As the oldest member of the Kawakami Family, Momoyo is seen as the older sister of the group. She also has a older sister-underling relationship with Yamato that she describes as a brother-sister type. As such, she'll occasionally pick on him, while also go out of her way to look out for him. She is bisexually flirtatious and will make a pass at girls only as she considers no males to be men due to her high standards, although defeating her in a fight, even if just barely, will make her think otherwise. Many people consider Hers and Yamatos relationship as something deeper and even describe it as interesting and special.
Source from Wikipeida