Komine Mitsuru

Working in the PR and Planning Department of a major apparel company, she became the protagonist's sister through their parents' remarriage.

She's smart and capable, but whenever she tries to be considerate of others, she gets flustered and ends up nagging and lecturing the protagonist, acting coldly. Yet her childhood dream was actually “to have a little brother.”

When her mother remarries and she gains a younger brother (the protagonist), she's overjoyed. However, being clumsy, she can't show it outwardly.

When she manages to act like a proper older sister to her brother, she rolls around in her room, delighted.