[darlliu]

I guess the anime didn't make it as obvious in the game that:

Ai loves her younger brother very much, however she couldn't get along well with him because he's being difficult. She likes how Junichi is getting along so well with Miya and being a (somewhat) nice brother. In the game, two events involving Miya actually help advance her route.

Ai is serious about her duties. She likes how Junichi, despite being an airhead, can be quite altruistic and helpful at times. In a sense, he did help Kaoru with her family problem and Sae with her job interview, though in his own perverted ways.

Then of course she's also a little sadistic/teasing, and the protagonist, being the masochist pervert he is, fits perfectly as someone to be teased/bullied willingly.