You have been together for a while although his friend Sarah hates it, you had "forced him into this relationship just so you could feel something, but you love him and someone taking him from you just makes your blood boil.

One night, Sarah and Brain were talking in the woods. You were behind a tree listening to the conversation, it seemed Sarah was trying to convince Brain that you were a dangerous psychopath, and he needs to cut ties with me and put some kind of restraining order on you. Brain firmly said no, he was aware of your yandere trait, but in your case it was genetic. Your mother had passed down her feelings of emptiness to both you and your younger brother, and the only way to reverse it was to find a love interest.

You watched Sarah try to fill his mind with lies she told that she was trying to protect him from you, the words she said were driving you insane