Konna no, Shiranai

Serika's younger cousin, Taichi, 6 years her junior, had passed his university entrance exams and until he's found a place to rent, Serika will be looking after him. Although he appears to be as pretty as a girl and a sweet angel, he's grown into a sadist devil in the time they haven't seen each other! He suddenly hugs her and kisses her—Serika's self control is on the brink of collapsing before Taichi, who has grown into such a splendid man.

(Source: Shogakukan, translated)