He's So Big That Only the Tip Fits!: Love With a Huge Height Difference

Yuto Shirosawa-Cromwell is half American and half Japanese, working part-time as a model. From the day he moved in next door, Ruri has been head over heels for him. She kept on confessing to him every chance she got and finally gets Yuto to accept when he graduates high school. However, it seems that Ruri's older brother, Kiyomune has made Yuto promise that he will keep his hands off her. They've finally become a couple, but is Yuto just going to keep treating her like his little sister...?

(Source: MangaPlaza)