Nanase Yuuko♀

Tomoya’s cousin how shows up later with her parents (the protagonist’s aunt and uncle). Catches the siblings in the act and schemes to turn Tomoya — who she’s had her eye on — into her plaything. A wildcard who does whatever she finds fun, including joining their escapades just for kicks. Acts like a carefree big sis but hides a sadistic streak, preferring to dominate. At first, she secretly joins the siblings’ play without tipping off the sister, but if exposed, she’ll drag the sister into a threesome for her own amusement.