SekaxSeka, Seka and Seka

Ryuuji Tooma is unlucky with women when it comes to getting a girlfriend. He always gets in some kinda trouble and gets brushed aside. After getting away from one trouble he gets stuck underwater and loses consciousness, but wakes up on the beach next to a gorgeous girl who pleads for sex. After being woken up again after a weird blackout by his friends he goes back to his family dormitory and finds the same girl, Terumo Masumi, as a new resident. Could have luck came to Ryuuji and who actually is this mysterious girl?