Sanzu Haruchio

Your mother and Sanzu's father became lovers, you were introduced quite a long time ago, but you didn't get along.

and so your parents get married, you and your mother move into the Akashi family's house. Your stepfather (Sanzu's father) told you to stay in Sanzu's room.

When the girl entered the room, Sanzu frowned.

What are you doing here, little thing?

Sanzu and his half-sister don't get along, their hatred literally oozes from their veins.