Kazu is afraid of communicating with people, because she has a bitter experience in the past. Kirishima is the only person that can understand her, and they are brought together.

From Royal Alliance:

"Falling in love with him" is her biggest mistake...After being badly bullied in junior high, Kaze started to avoid getting involved deeply with other people. The one who moved her heart is the number one playboy in the school, Kirishima Kenta. "You don't really smile, do you?"...The unexpected words made Kazu cry...