"My Guy" syndrome is often misused as a term. It's when following your idea of your character's persona creates insurmountable problems for the game, usually exacerbated by the GM being a weenie and not just saying "Well, since your PC moved to Montana while everyone else is fighting vampires in Chicago - you're going to have to roll a new character to get into the actual game I'm running." MGS is derailing the game in a way that's not fun for the group. It's not just "roleplaying in such a way that values something other than overcoming maximum combats per unit time" or "advances the plot down the railroad without any deviation." Roleplaying and roleplaying conflict are not My Guy Syndrome.

In this case, this situation sounds like just plain ol' roleplaying and drama (in the good sense). You have a game which, it sounds like, accepts intrigue and PvP secrets (if not overt PvP action) as an OK part of the narrative - like many games do nowadays. My group is fine with this; some of our campaigns do reach a point where characters with different agendas finally reaching a breaking point and have to work through very different approaches to a problem.

In many games, where PvP is allowed, the scenario you depict isn't a problem at all, and them killing your GF and you knifing one of them over it is fine and a dramatically appropriate part of the story. You've said your group generally doesn't do that - but maybe Strange Aeons is the time to do it - you were all bad to begin with and are all going crazy from Call of Cthulhu stuff. I would venture to say "putting a fellow PC down" is a pretty stock part of CoC games, even if it's a topic of debate in many D&D groups. So it's worth getting an understanding with your DM and other players if it's OK for this game to go there.

Now, if you think that this is going to seriously turn into a problem - like if the other PCs consider whacking your GF OK but the idea of your character fighting to stop them or something would be socially unacceptable - then you do need to handle this with them more explicitly. You can try various in game dodges (rings of undetectable alignment etc.) but that's just taking some steps to kinda hope this doesn't come to a head, and you can't count on any of that working 100%. You can't always handle things like this purely in game, since you're talking about solving a meta problem (player feelings). You may have to sit down and talk with them, and say "hey guys... you have to know my character isn't going to lay down for you whacking this chick... I'm fine with me getting killed in the bargain or whatnot, it'd be a cool death, but if that makes you uncomfortable we should talk about it." Or whatever your feelings on the subject are. If you feel like there's no way you can character/plot justify your character continuing to associate with these people if they did that, then you should go ahead and tell them "I'm going to have to reroll if this happens." They may not know it's a big deal to you, may not put much thought into their character's psyche, etc.

It might be a tough sell, but have you tried asking the other players to abandon their murder-based plans for the sake of making the game more enjoyable for the group?

As your describe the situation, your character's motivations are at odds with theirs. That does sound like My Guy Syndrome, and the best solution is usually to have the player at the center of the problem "choose to act differently."

But that's not the only solution - it's just the one that's usually easiest to implement, because characters usually have other options open to them, and can just choose a non-problematic one. It's not always the easiest solution, however, and it sounds like this might be one of the rare cases where it isn't.

Love, as a character motivation, is almost impossible to set aside; Very few characters will assent to the murder of a loved one - and the rare few who do are unlikely to continue associating with their significant other's killers. That means that even if your character is the one to act differently, allowing the other player characters to kill his or her girlfriend stands a risk of driving your party apart.

When the campaign and party are in danger of falling apart due to conflict between players' expectations, it is definitely time for an out-of-character discussion with your group, and an alternative solution, and one that doesn't threaten to tear the party apart, is for the the other players to decide that their characters act differently, instead of yours. Like I said, this is going to be a hard sell. Still, it is a solution - and one that preserves your fun, and that should be possible without compromising their characters' motivations; After all, very few people are willing murder their friends' loved ones, any more than their own.

Whatever you do, however, do not make the discussion an ultimatum. Tell the other players you're having trouble finding a solution to the problem, and solicit suggestions; Propose "not murdering the girlfriend" as a possibility, but don't say it's the only possibility: Just that it's the only one you've been able to think of so far that seems like it would work.

The other players will mostly likely understand your concerns once you've explained them, and will most likely make helpful suggestions of their own; Perhaps they'll be willing to let your character's girlfriend be a double agent, or an informant, or a saboteur. Even if they don't agree to an alternative plan, they might be able to suggest new perspectives on the situation or new plausible ways for your character to react that would make their murdering her without breaking up the party more feasible.

Whatever the outcome of the discussion, act with grace, try to enjoy the game; If the discussion goes in a way you don't like... Well, maybe you'll have o accept it, and live on. Maybe the group was playing two different games the whole time, and you just didn't realize. Maybe the best option will be to retire your current character and introduce a new one; Leaving the party for personal reasons could be an interesting character development in itself.

Good luck.

What would you do if this were real life?

I can think of a few possibilities:

• Think about if the character is evil, despite your love. Can you come to terms with the fact they need killing? Think about the ending to Of Mice And Men.
• Can you alert the authorities? If the players want blood, then alerting the local militia of their plans would put them on the hook for murder. They may then go for capture and fair trial, which is a much nicer option.
• Can you alert the evil character to the player's intentions so that they can flee?
• Can you explain the fact that you will die before you let them kill her to the party/GM? They may act differently, or the GM may engineer a solution that doesn't result in your collateral death.

It sounds like a puzzle to me that you need to try and solve. But you don't always win.

Rule #1: Never point a gun at something if you're not willing to pull the trigger.

If you're not willing to kill your fellow PCs, then your character should behave in such ways that your character doesn't end up pointing a "gun" at other PCs.

If you ARE willing to kill your fellow PCs, talk to your DM, and then your whole group, about the nature of inter-party conflict in this campaign. That'll inform you as to how much gun-pointing can be done without causing interpersonal problems.

You only control you: Part of the problem with "My guy" Syndrome is that such players aren't just controlling their own player, they're controlling the overall gameflow in a way that denies player agency to their fellows. The logical conclusion of one player not wanting to kill a villainous NPC and another player wanting to kill the same NPC, in a game with no inter-party conflict, is that the NPC ends up dead. That's a hard assumption of playing with Paladin-types without party conflict. Accept that, and negotiate within that framework.

Point the gun at yourself: If your character adamantly believes she's redeemable, run a "my life for hers" or "my life with hers" story, and see what the Paladin does. Tell the player outright that, per my first point, above, you've planned for and accepted that his Paladin might kill your character, and that's FINE. It'll be a cool story, though. It'll add depth to the campaign, but when the party Monk beats you unconscious instead, you'll add even more depth, and you haven't removed the other players' agency, cause they can still choose to kill the NPC.

Great question, and a number of great answers so far. Let me add one more.

Suggest to the party a "let he who is without sin cast the first stone" sort of argument. You mentioned you all had lost your memory, but suspect you too were part of this cult. Shouldn't you all be equally guilty as the NPC for things you did, which you just don't remember? Or, to put it an other way, if wiping your memory of your own horrible past lets your party skate with a (now) generally "good" alignment, why can't she get the same redemption?

You don't know why you lost your memories, but if she chose to do so, as a sort of penance or repentance after you escape, I'd argue she would be in a superior moral position than the rest of you. She choose to change and be good, you had goodness thrust upon you, as it were.

If this an acceptable solution for the party, you may want to have a meta-game discussion with the GM about having the NPC produce some proof of the misdeeds the party did before their memory loss.

If the out-of-character players are up for it, this sounds like a great set-up for some fantastic role playing, how ever it turns out.

These are rough waters, beware the icebergs

Mike's answer is an excellent perspective shift away from just having the two options of kill her or let her go. There is the redemption angle, the double agent angle, the "only there for a friend" angle, and possibly more. But there's a problem.

The "friend" angle is out

Your latest edits have removed the "only there for a friend" angle because the "friend" is the cult leader, and removing the cult leader is the goal. The friend cannot be saved, and thus cannot be removed from the picture.

The "double agent" angle might run you into trouble

There is also the issue of party alignment, if they would even greenlight the idea of using an Evil character to get their way via being a double agent. For the Lawful Good Inquisitor, this might be a problem, and you may not see eye to eye. Ned Stark will live by a Code of Honor, even if it kills him. Also, your party has to trust her to use her as a double agent, and you imply this is a hard sell.

Her betrayal is very likely going to happen if you don't know her goals

Another issue is her motivations. You have to think about why she is helping you but still staying in the cult. She might reciprocate your feelings (and those feelings are strong enough to save you), but chances are, she has legitimate motivations for staying in the cult other than "just" to help a friend. No Evil character would do something that nice. It is either she is blackmailed and cannot leave if she wants, or she has something to gain by staying. Do not assume there "might" be a betrayal -- because there will be. If she doesn't betray you, she will at least betray your party mates, while offering you and you alone a chance of escape.

The "redemption" angle is probably not going to work

Finally, it is a hard sell to try and redeem her. You can stall, buy her time so that the party does not kill her outright, but if she is fundamentally Evil and cannot be redeemed, then the story of her redemption is just a story, and in the end you will be lying to your allies and putting them in jeopardy. If you offer the "make her surrender" angle, you can safely assume she will not want to surrender. If she has killed as many people as you imply she has, surrendering is her one-way ticket to capital punishment. This story breaks down as soon as it runs into reality.

Conflict is a two-way street

What you have to remember is that conflict is a two-way street. If you're worrying about just how to handle your party mates, you're missing that her motivations are also in conflict with theirs, and any plan you make has to take into account her motivations as well. If you cannot present a win-win solution to her and your party, and if neither side is willing to yield, then one side will have to lose eventually.

Find out what she wants before you make a plan

Why is she helping you? Why can't she leave the cult? What has been promised to her if she stays, and what will they do to her if she leaves? Is there something she wants that is driving her forward (a reward like power or gold, something that doesn't require a true belief in the cult to attract her), or is she running away from a consequence?

Your character, while in love with her, does not seem to know her. You know your party mates, but they are only one half of the equation. Find out what she wants before you make a plan.

This is important because there is the possibility that her plans, and your party's plans, may not be mutually exclusive. That is, it's possible she can achieve her goals while your party achieves theirs, both at the same time. Or they might not. The point is, you don't know that right now.

Where there's a will, there's a way

It might involve deceiving your party to save her life, but it beats outright leaving or trying to kill them. Almost any goal she's going to have will probably not be mutually exclusive with what the party wants, unless her goal is "to usurp the cult leader" or the party's goal is "to kill the NPC you love."

If her goal is to get an item in the cult, then acquire it, give it to her, and stage her death/escape.

If her goal is to prevent her family from being killed (because they're held hostage, for example), track them down and free them.

If she wants power or gold, convince her that there is a very powerful group (you and your party) who is out to destroy this cult. Trying to achieve power or gold this way is a doomed proposition, and she should abandon ship and find something new.

Once you have the full picture, you will be able to create a plan to save her, while also not betraying or leaving your party.

The GM clearly is trying to play up/into the situation.
(...)
He also does find the interaction with the NPC to be one of the better established character motivations/roleplaying hooks.

(...) the GM even let me roll that Sense Motive check so I'm pretty certain.

This is an excellent opportunity to have your character find out where his true loyalties lie. Whatever the outcome, it will give a new dramatic depth to your character. This is quite possibly the reason the GM set this up.

Let's assume we don't want a situation where one of the PCs is killed.
Also, keep in mind that the NPC is in the hands of the GM.

How sure are you of the outcome of that Sense Motive? Perhaps your character has picked up false information. Or perhaps he picked up partial information: her motive for loving your character is true, but she still feels loyal to her cult and is going through the mirror image of your character's difficulties.
Since she's controlled by the GM, you don't know. When it comes to a confrontation, you may be in for an unpleasant surprise.

You only control your character. Where do his loyalties lie, ultimately? If When the GM pits the party against the NPC, what is your character going to do?

Prepare for several outcomes.

Choose your party

If your character chooses to remain loyal to the group, he can remain passive while other members deal with the NPC, creating an interesting but destructive intra-party dynamic. ("You killed my girlfriend! And I let you.")

But your character can (and perhaps should) play an active role. When he finds out he was betrayed by the woman he loves, or when he finds out that she expects him to turn evil, he may come to the conclusion that the only option left is to fight and quite possibly kill her.
If he does kill her, that gives your character a new emotional depth. ("I had to kill my girlfriend because she turned out to be evil — we truly live in an evil world.") If he fights her but doesn't kill her, it may be the beginning of a new and powerful foe, perhaps even arch-nemesis. Both for your character personal and for your party as a whole. This may very well be what your GM is going for.

Choose your girl

I don't think this is what the GM wants, unless he's set to destroy the group. So if you think your character should really choose his girlfriend, even if she is revealed to be truly evil, talk this through with your GM beforehand.
If he (and the rest of the group) is willing, you can either have your character split, going solo for a while. This may even involve having your character join the cult.

Remember that you only control your character; you do not control this NPC who says she is your girlfriend.

All these answers make fine points and generally are great reads for navigating this situation. The way I would respond to myself now having read some of this and having more time to consider the matter:

First, bring this up with your GM

A matter like this needs to be discussed with the GM, before bringing it up with the party. When talking with your GM you should be safe enough to openly state that this could turn into an ultimatum to the party. It is partly their responsibility to referee player disputes that might arise, as well, knowing what a player might do helps inform them how to structure encounters and NPC interaction. It could be that the GM can find a way to prevent a situation where your PC feels cornered into betraying the party. Perhaps the love interest NPC gets killed by the bad guys for acting as a double agent, maybe she just turns on your PC, or perhaps she finds a way to lie low while the party does their thing and you reunite once the smoke clears.

The GM also should have some stance on how they would want to handle potential betrayals or party PVP, and you should listen to their advice and work with them to produce whatever dramatic outcome is desired.

markovchain's answer makes some good points that a GM can consider. For the GM, fleshing out the motivation of the love interest NPC can help provide clarity on how to proceed.

Second, understanding that this isn't really "My Guy Syndrome"

This is roleplaying, often one of the main reasons why we sit around a table and roll dice instead of just booting up some video game that operates off the same mechanical principals.

Over the course of the game your character has developed a series of motivations and relationships that have moved the character to the point of being willing to turn on the party in the event that they turn on the love interest NPC. This isn't some idea for a character trait you're just running with to a damaging outcome. This is you playing your character in the world the GM has presented, and arriving at a meaningful and solid in-character response.

The very act of realizing that this feels like it might be MGS and asking for help on this means that you don't want to just blame your potentially hostile action on your character.

Third, talk to the group

Talk to your fellow players, be open that this is important to your character and implore that they don't push this. Don't phrase it as an ultimatum, but rather that this is a goal and desire of your character. It might be a tough sell, but it sounds like there are rather few players with a real opinion on this.

Fourth, make a call

This isn't an enviable position, but on some level you the player are going to have to make a choice. Recognizing this as a choice means this isn't MGS. Either way, this choice should have a meaningful impact on your character, and maybe you will need to retire them. Whatever you choose though, own it. Don't let your friends push you in one direction, and just just simply think "My Guy is going to do XYZ." Make a call that is going to make the game dramatic and fun, for you, the players, and the GM.

To those interested in what happened after all of this...

First, I brought this up with my GM in private. I frankly stated my concerns for what was going on and where things might of been going, openly saying that if it came to it, I would be retiring my character, and likely handing him my character sheet.

It turns out that a way of approaching this situation was already in the works with the GM, and he provided a situation during down-time (the party was in the process of looting a fort we had taken back from the cult's control) where the love interest NPC was able to make contact again. She had assumed a disguise as one of the ally NPCs the party had been working with.

From there, discussions on future plans were made. With some effort (my character proposed to her), it was possible to convince her that her best friend, the current leader of the cult, was likely a danger to herself and us. My character committed to trying to reign in the cult leader without killing (so of course, a new problem to run by the group, "Hey guys, I want to take more risk during this boss fight and try to take them alive, for reasons you wouldn't possibly accept"), if possible, and the love interest NPC was willing to go along with that. She then said she would hide out and wait for this to blow over. Which, if this is true, would mean that I should be able to avoid a situation where the party actually sees a need to attack her, as she won't be in the primary cult compound. And I can handle this later on my terms if it needs to be kept secret from the party.