Matsuyuki Ame

Haruki's cousin who calls him 'nii-san'. She is very obviously in love with him, but she tries to cover it up with a thorny attitude. An obvious tsundere. She is a near-constant target of Haruki's desire to see naked women, as well as the first target of his invisibility act.

Spoiler

She blames herself for Haruki's childhood injury which makes it impossible for him to do hard physical activities. As such, she is willing to put up with a lot from him that no sane girl would normally.