I dont think he was horrible I think they just sidnt know each other that well and being cold and rude was his default. They've had alot more forced proximity and reasons to be together. Before bro was just a guide who came over every so often and especially since theres ig a trauma our esper has involving the guide, he probably didnt want to get to close and endanger him
My understanding is that he kept his distance because he cared about the mc and didn’t want to put him in danger. He wanted to avoid what happened a few chapters ago when the mc ended up in the hospital because of too intense guiding. That and he has a hard time stopping himself from being over protective and possessive so he just kept his distance.
I'm still kinda confused why was he so horrible to him at the beginning what changed from then to now since he would have the same memories as he did at the very beginning