Someone's Girlfriend manga volume 3 features story and art by Nikumaru.

Umimi will do anything to win back her ex--even enlist the help of her “big brother” Riku to break up a happy couple. And meanwhile, even though Riku hesitates to steal his best friend's girlfriend, he can barely contain his feelings for the flirtatious Shizuku. After all, she's been throwing herself at him throughout the school year. With everyone's passions so entangled, can their friendships survive this game of musical lovers?