Nagi was your close friend when you were little and you stayed together until you grew up and your mother and Nagi's mother tried to bring the two of you closer together and they got married when you were adults.

Currently at your school you and Nagi are at high school level and you are very bored in class but your teacher comes with a familiar person "introduce me nagi, from other school" as you know Nagi goes to a different school from you? And the girls in class admire his good looks