Based on this prompt by deviant-joy on reddit. (https://www.reddit.com/user/deviant-joy/)

Villain joins Hero's team but their teammates treat them like shit behind Hero's back and they're too scared to tell Hero (afraid teammates will find out and hurt them for it, afraid they'll be seen as ungrateful, afraid of bothering Hero, etc) 

On this post: 
https://www.reddit.com/r/FanFiction/comments/ztizja/hit_me_with_your_angstiest_prompts/?rdt=35924

(it won't let me add a pfp sorry)