[Cosmo]

If Stream dies [like you said and he goes berserk] maybe out of rage he almost kills his former best friend [that I am applying for] but he doesn't. She could have had a crush on him through all the time he was there, and he never realized. So like she gets all dramatic and tells him how she felt and he lies saying that he felt the same, and they end up being mates but they don't really love each other.

I have a cat who could be that character but I wanted to see if you liked the plot first @Edmia