Uhm, since I'm writing with my friend some RPGs... Or, well, I try to do so, but her characters always are annoying me pretty soon. They are not total Mary Sues/etc but nearly and my characters become completly unimportant/ignored and hers are becoming completly overpowered.
Slowly I start to become angry because of this, so I guess I should tell her how I feel.
But I don't want to hurt her or make her angry, since she's pretty sensitive.
Does someone have an advice how I should handle the situation? :/ I really dont know what to do anymore...

Be blunt and tell the truth or suggest changes to their op character.

Yeah, I guess thats the only option... *sigh
Thank you

Oh, Mary Sue-esque characters I'm afraid are the worst kind so I share your fury, heh. Yeah, it'd be good to tone it down a bit, and communicating that to her in whatever way you deem appropriate without completely alienating her would probably be best as you said. If she's doing it on purpose then you could just reason with her probably, but if it's on accident, well, just explain your POV, and as your friend she should understand.

Here, this might be able to help your friend out~ :D
http://tvtropes.org/pmwiki/pmwiki.php/SoYouWantTo/AvoidWritingAMarySue

I UNDERSTAND YOU BRO. YOU HAVE NO IDEA. I literally just said that I wasn't going to role play if the characters were going to be unnecessarily OP or "different." (For example; Mental Disorders, different eye colors, Tattoos even though they have been in stasis since they were five...) I understand. Just tell them it needs to stop.

>>4 at first: omg your name is so yummy Q-Q
Thank you for the link! I'll try it out >w>5 I always have hope that it isnt going to be annoying.. But it always is. *sigh
But I'm glad someone can relate to it :D
I have to find a way to say it nicely and to explain whats so annoying.. But its difficult ><

>>6 Lol thanks I seem to get that alot. People like to eat me T_T

Anyways, yes, I think that list should help for the most part, and if you want to ease her into her intervention of sorts, you could show her examples of awful Mary Sues. I could dig some up, even; Jenna from My Inner Life and the My Immortal person are two egregious examples that come to mind and make me shudder with revulsion. In any case, if you need anything else, ask; TV Tropes is a great resource for writing in general and I hope you can get even more use out of it than just that index :D

Yikes, this brings back bad memories. I used to have this exact same problem. I never said anything to this person about it, and let me say that never helps anything.

You should try and bring it up gently though because your friend may not know that they're doing this. I feel like the kinds of characters people write are influenced by what they watch or read, so maybe you can suggest some new book titles or tv shows with better characters to give your friend new ideas.

Definitely make sure you bring it up somehow though, otherwise you'll probably just get more and more irritated, with stories becoming increasingly one-sided, and that kind of one-sided compromise isn't good for anyone.