Missile Happy

Summary from MangaUpdates:

Mikako is 15 years old and she lives since the death of her parents alone with her elder sister Megumi. Megumi wants to make an Omiai (a wedding with a man, she doesn’t know; an arranged marriage). But Mikako has a sister-complex and wants to look at him by living in the same apartment (”through a mistake by the broker”). Mikako is shocked when she sees, that this BOY is probably only 17 or 18 years old (or even 16..!). Too young for her sister! Mikako thinks, that Ro is a playboy but she discovers that he fights for his dream (his father wants Ro to become his succsessor, but Ro wants to become a physician and he gets no financial help from his parents). While living at Ro’s apartment, Mikako slowly fell in love with him…

Title: Missile Happy
Author: Kiritani Miki
Status: Complete
Genres: Shoujo, Romance