What you have is a clash of expectations. Your group likes to plan and strategize, she clearly doesn't (or rather, planning and strategizing is less important to her than playing this impulsive character). It's really important to articulate the expectations of play and you should do talk to her, and the group, directly, rather than create an NPC to try to block her character.
You'll notice if you look at other media or stories, there's a big difference between impulsive characters who TALK about jumping in, but don't actually do so, vs. those who DO and cause a lot of chaos.
If your group expectations are that everyone is mostly working together and only conflicting/at odds for show ("The elf and the dwarf are arguing again." "Oh, let them be, they're actually good friends.") and she is operating on the idea that each character might have very different agendas or less concern for the group, you're going to have clashes.
Being clear about what kind of game this is, helps her decide if this is the game she wants to play or not.
Disclaimer
- Your ability to constructively address things like this as a player will, perhaps unfortunately, depend on how central or peripheral you are to the group socially. And this is definitely a social problem, not just a game problem, so I'll be attempting to channel Captain Awkward for much of this answer. But I wanted to start out by admitting the possibility that you may not have "standing" to address this issue other than bringing it to the GM's attention, which it sounds like you've already done. I hate to say it, but this may be especially true depending on the nature of your relationship with the GM; some people will make assumptions that could make it more difficult for you to bring up issues, unjust though that may be.
- Note that I don't think that means you shouldn't say something. The GM's role in "running the game" depends on the system and the particular game, but that need not and often should not translate to running the gaming group. Some situations are customarily addressed by the GM as an extension of their role as referee, but in real life the GM isn't in charge of anyone else, and it's unfair to always expect them to solve table problems - mayking the game fun is everyone's responsibility.
- Most of the below is written based on the assumption that the problem player here is basically operating in good faith, but frustrated with how things are going and expressing it badly. This may not be true - frankly, dude sounds like a pain, and you could choose to be more assertive - but there are reasons to at least verbally leave open the possibility:
- Having been the guy who was doing something wrong and didn't find out until later, I really appreciate being given the benefit of the doubt.
- It can make for smoother, less confrontational conversations.
- It's kinder to give people some credit/dignity, even if they don't deserve it, which is more important the more invested you and others are in maintaining the relationship.
- It will often make it pretty obvious if they are operating in bad faith and don't care about your feelings, which is disappointing but makes it easier to do what you have to do next.
The Simple Way
Don't play with him. You're there to have fun, and he's making things not fun. If your comment about what he wants is accurate, and that would indeed ruin the game for the rest of you, then there's probably no way for everyone to get what they want, so majority rules. Of course, this is simple in concept but not necessarily in execution; still, it's preferable to letting resentment build into an explosion that leads to the same result but with even more hurt feelings.
Now, normally this would be done by the GM, in a private conversation, but if they agree it needs to happen but don't want to do it themselves for social reasons, it could be easier if done as a group, and you could start that conversation. Probably after or towards the end of a session, when he's been up to his usual shenanigans, I would call him on it and say "Hey, it really makes it less fun when you do X, and you don't really seem to be enjoying the game that much yourself. Since we seem to have such different styles, is it possible this specific game, with these rules and expectations, isn't really your thing?"
Note that this doesn't have to be personal, though it's difficult not to take something like this personally, partly due to Geek Social Fallacy #5: the idea that friends do everything together and therefore not wanting do do a particular activity together must mean you're not friends. To mitigate that, you could suggest specific, reassuring alternatives: "I don't think this game is working out as is, but maybe you and I could do [ACTIVITY] on [WEEKDAY] instead?" Don't suggest that if you wouldn't actually enjoy it, but perhaps you could arrange for someone who would to do so.
The Complex Way
Try to resolve the situation in a way that lets you keep playing this or a similar game. Ask this player to help you understand what makes them act this way - why they do the things they do, and why they respond the way they do to your actions (because the examples you gave really, really don't sound like any definition of metagaming i'm familiar with). This will be a long, possibly multi-part conversation. Two broad categories of possibilities, likely both present in some proportion:
- Something is going on in their personal life that makes them easily frustrated when things don't go their way; either a generic stressor or tension in their relationship with another player(s). You likely can't solve the root problem at the table, but pointing it out could help them realize they need to keep it out of the game.
- They want to play the game one way, you all want to play it a different way, and most likely, this gap was not identified, discussed, and resolved before play began. Different players are motivated by wildly different things. It may be possible to craft a game that meets everyone's needs at least some of the time - a little character drama here, a little violently eviscerating anyone who stands in your way over there. This is mostly on the GM, but also about character design - sometimes having the more fight-y players play more fight-y characters, like bodyguards, can ease the tension. They're still not talking much and still more interested in killing, but now it's in character. Heck, sometimes just the act of pointing out that there are multiple equally valid ways of playing games by itself can help, if part of the frustration comes from a feeling that the other players are doing something objectively Wrong as opposed to "not what we're going for this time."
If you can't find common ground after working on it for a while, you still have The Simple Way, and at least you know you tried.
Either Way
1. Focus on a specific, recent incident or two. You can allude to the fact that it's becoming kind of a pattern, but "You always..." is not the start of a productive conversation. Presumably, this player hasn't been thinking about this issue as much as you have, so a narrow, concrete focus will help them have information to work with.
2. Use I-statements. Saying "You need to stop doing X", while very possibly true, can sound too much like "Your behavior is bad and wrong and so are you."1 Saying "I really thought I was playing my character, and I wasn't out to get you, so I was hurt when you accused me of that," highlights the effect their actions are having and invites them to reconsider.
3. Avoid an "intervention" vibe. Especially since you're not the GM, try not to speak for anyone else; just mention your issues with this player's behavior. There may come a point in the conversation where it's appropriate to invite others to share their opinions, but anything like "We've decided..." sounds too much like "We've all been talking about you behind your back, and the conclusion is that you suck!"
1This point applies in cases of mildly annoying behavior where you're trying to keep things friendly. When the stakes are higher, like if the behavior is making people feel unsafe, it's much more important to be clear about "You need to stop doing this, now." I'm interpreting this situation as the former, but I could be wrong and either way I wouldn't want people to generalize too much from this point of the answer.
Best Answer
Every once in a while I have things like this pop up, too, these one-sided running "jokes" at someone else's character's expense. I have never found anything that is effective except for telling the other player, quite bluntly, that their behavior is annoying and disrespectful:
"I am not here as a prop for your jokes. My character is not there as a prop for your character's jokes. And something that was (charitably) funny once becomes much less amusing on every forced repetition. This wasn't fun to begin with, it isn't ever going to be fun, so just knock it off."
You should not need the GM's support in this matter, but it is always quite helpful if you can get it. (On the other hand, if it turns out you do need the GM support because the player won't stop, and you can't get it, that is a major problem.)
I've also generally not found it helpful to turn this into an in-character or rules-based dispute either, unless you are so inclined (and it is clear from the above that you are not.) As you say, in this specific instance, the likely resolution in-character would be violent PVP. But even if it were not, if there were some less violent avenue for your player to take, the fundamental problem is still there: Your character is a prop, the butt of someone else's joke, and now you have to spend your precious time and role-playing energies in constantly fending this off. That's just not fun.
This answer has some important insights, too, which I think are complementary with mine. Not all these running "jokes" are borne out of malice, although they can still be quite annoying. How much the cause is malice, or awkwardness, or misreading the situation, or some other thing is up to you. And this can and should inflect how the blunt response is delivered. Blunt need not mean cruel or embarassing.
But that said, even in the awkward/misread situations, it has been my experience that the player in question needs to be told, clearly and firmly. They're probably not going to figure it out on their own.