My sister's Midori-kun

This is the beginning of the bittersweet first love. Sui doesn’t really get along with Midori-kun, who’s her sister’s boyfriend and a college student. He always treats Sui, who's seven years younger, like a kid and teases her. She feels it’s so annoying! But after he started tutoring her, an unexpected side of him gradually began to show..? (Source: MANGA MILLION)