Someone's Girlfriend manga volume 2 features story and art by Nikumaru.

Umimi returns to her hometown to find things have changed quite a bit. Her old junior high boyfriend, Takuto, is now dating the gorgeous Shizuku. Meanwhile, her senpai, Riku, is clearly struggling with his feelings about his best friend's girlfriend. In order to win Takuto back, Umimi needs Shizuku out of the picture, which might give Riku exactly what he wants. But are things ever that easy when Shizuku is involved?