Sekai de Ichiban Ani Shiteru

Yori, a 2nd year high school student suddenly finds herself living with Seki, a 2nd year university student who's the son of the man her mother just remarried. When Yori arrives at Seki's apartment, the person who answers the door is half-naked and a little scary... but for some reason, he immediately starts doting on her!? He's frivolous and overly affectionate... but she just can't hate him.

(Source: Kodansha, translated)