Kokoro Logic

Natsume Aoba a young man who is going out with a guy named Onozaki Yohei. Yohei though has a thing called “multiple personality disorder.”
There is Yoji, easy-going and only does what suits his mood.
There is also Yoko, a female personality. Very elegant and has common sense.
Last there is Yoichi, very selfish and arrogant… he is the most troublesome.
From Baka-Updates:
Aoba is dating the sweet, puppy-like Youhei, but he’s also dating the possessive and domineering Yoichi, the elegant and refined Yoko, and the easy-going Yoji, because Youhei has a multiple personality disorder! Aoba is trying to convince Youhei that he loves only him, but Youhei lacks confidence and with Yoichi constantly interfering will the message ever get across?