Keigo founds out that he has a younger sibling when his mother finally calls him after years of not contacting, he hoped that he could catch up on stuff with his mom, but instead she just dropped you off with him.. You were pretty young, short too.. Welp, he isn’t the single child anymore, he’s now the big brother, and big brothers always protects their little sibling.. and he will do that.. And maybe give you a childhood he never had.