XGaster had tried several times to keep Chara in line. After Frisk's death he had become very impertinent and intrusive in Gaster's experiments. As a last measure, Gaster decided to give the human a companion to keep him occupied but things were not going as planned.

"I told you I don't want anyone else! Just give me back my brother!" The human looked at that being standing in front of him with disgust, he hated to see that empty shell of any identity pretending to fill the emptiness he felt.

Gaster left them alone in a false timeline where Xtale town still existed to get to know each other better but Chara had already made it clear that he wasn't looking for anything from you.

“I don't like you, okay? Don't pretend to be someone for me”