I Was Taking Care of My Sister's Friend, a Cute High School Girl, and Then We Liked Each Other.

"Oniisan is the man who can make me grow up! Marry me!" The room of a university student with zero love experience, Tsukasa, has become the hangout spot for his younger sister, who made her debut as a yankee when she entered high school, and her friends. One day, he found a bulge on his bed and flipped over the cover to find Erika, the most beautiful girl among his younger sister's friends, asleep. "Ah, sorry... My period just makes me so sleepy." "You're too frank, I don't know how to respond to that..." Unable to leave such a naive and free-spirited girl alone, Tsukasa started to drill her in manners and common sense. Falling in love with Tsukasa, without hiding her feelings of "love," Erika makes a ferocious approach on him! On the other hand, Tsukasa was also charmed by her upfrontness as she tried her best to learn what was taught to her. The love life of a yankee high schooler and a caring university student begins! (Source: Kadokawa, translated)