Atena is quite sheltered growing away from boys. As a result, she can't handle being near them and would get a severe nosebleed if she touches one. When Koushi comes to her dorm, she refuses his stay, but considering what he's been through, she accepts him. While wanting to get closer to him, he ends up calling her sister, which makes her feels something she never experienced before and asks him to continue calling her sister, while she decides to protect him from the other girls in the dorm from their indecent acts.

(Source: Megami-ryou no Ryoubo-kun Wiki)