"Why? Shouldn't a yandere try to be someone that -could- win their beloved's heart?"
I should probably clarify that the reason Paris dresses and acts how he THINKS the MC would like is because in the current demo he doesn't have any feelings for her, just a passing fancy. He dresses like that because a certain person in his life dresses and acts that way and they are in a relationship. So, he believes that if he "becomes" them he'll get a girlfriend. At the moment he just dresses like that because he thinks that's the kind of thing he thinks girls are attracted to. He will very quickly develop an obsession with the MC and you can kind of see hints of that creeping in with the questions he asks you in class.
For picking your interests and what you like the game kinda already has that? When Paris asks you questions, for example, he asks you what traits you like in a partner. I'm planning on whatever you chose for that question affect his personality IE: If you chose you like a more affectionate partner he'll be more affectionate towards the MC, if you chose you like someone who's protective he'll act more jealous in the protective sense when you hang out with a different love interest instead of him. Like trying to make you think that the other love interest is a bad person.
Hope I was able to clarify that!