Ouka Shiunji is one of the third oldest Shiunji siblings, along with her "twin" brother Arata Shiunji.

Ouka is very popular and outgoing, easily able to make friends of any gender. She is the ace of the school swim team while simultaneously ranking as one of the top students in their entire school, second only to her older sister Seiha Shiunji. While kind hearted to most people, she tends to be rather feisty with Arata as the "twins" regularly butt heads. Despite this, the two share a close bond and it's stated that they had many moments in their childhood where their thoughts and actions were so in sync that friends assumed that it must be some kind of twin psychic connection.

(Source: The Shiunji Family Children Wiki)