I'm Underage But I'm Not a Child (2017)

On Karin Oriyama's 16th birthday, she receives a present from her parents. Her present is marriage. The person she marries is her first love Nao Tsurugi. He is a popular senior in high school. For Nao, he only marries Karin for economic reasons and in particular the wealth of her parents. Karin loves him truly, but Nao doesn't love her at all. Their marriage is a secret, but Isuzu Ebina, who holds feelings for Karin, learns of their marriage.