Kisaragi Yuu

Yuu is the younger brother of Maaya. All his schoolmates say his older sister is a kind and beautiful girl with amazing boobs. He actually thinks the same way and considers his sister to be the cutest girl in the world, but he doesn't like his friends looking at his sister's breasts and at times he becomes very protective.

He has always been very close to Maaya, who has always been very kind to him. They used to do absolutely everything together, including bathing. But his sister's actions had begun to trouble him as she kept exposing herself more and more. He was completely fascinated by his sister, and not only had he begun to find her to be very pretty but also very erotic.

He had recently begun to fantasize about Maaya while masturbating with the help of his friend's eromanga, but she caught him in the act as he was moaning loudly while exclaiming "onee-chan". She asked him whom he was calling while he was relieving himself, the magazine's onee-chan or she, Maaya-nee.

And this event marked the beginning of their story.