MC is placed into a novel/story he read about an abused protag as the evil brother. He has a system in the shape of a cat and is tasked to act as the antagonist despite adoring ML. ML, unknown to everyone, can literally read people’s thoughts as word bubbles. As such, he comes to realize that the brother who seemingly hates him is constantly showering him with love and concern. ML develops a crush on MC and wants to pull him from his strange behavior. Not sure if it's Chinese or Korean but definitely one of the two.

I'm no angel, but I'm more pure than you.

Is it I Want to Be a Big Baddie
Not sure thou

Ordinary girl who loves handsome + tall + black-haired + rich + mysterious + cool + possessive / obsessive / yandere / overprotective / stalker + gentle (ML/Seme)

Yes, this is it. Thank you so much~

I'm no angel, but I'm more pure than you.