Sekai de Ichiban Ani Shiteru

Yori, a 2nd year high school student suddenly finds herself living with her mother's new husband's son, Seki, a 2nd year university student. When she visits Seki's apartment, a half-naked, scary-looking man comes out... 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)