Masato Sendou

The second son of the man who got married with Haruto's and Aki's mother after the divorce, Takahisa's little brother, Haruto's and Aki's little step-brother, due to mentioning Haruto in front of Aki being taboo he was never told that Haruto was his older step brother living away with his own father, the first day of his sixth year at the elementary school he got dragged into Takahisa's and Satsuki's hero summoning.

(Source: Wiki)