Sei-chan Capacity Over desu!

Futaba Ito is just about to start her first year of high school when Seiichirou "Sei" Izumi, her childhood friend, decides to move into the Ito residence because of his overseas-based parents. Sei is quite the charming young man and two years older than Futaba—the same age as her elder brother, Tsumugi. Naturally, Tsumugi is wary of leaving Sei and his sister to their own devices, and he has a quick chat with Sei to assert that he won't tolerate any shameful behavior. Sei agrees―but teenage zeal is a difficult thing to control. Every time Tsumugi looks away for too long, Futaba and Sei get a little closer, exchanging smiles and holding hands discreetly. Thanks to their cute little escapades, Futaba finds herself helplessly falling for Sei. But as their interactions rapidly grow more intimate, Futaba wonders if Sei's affection might just be too much for her to handle!