Love Love Boin Takabisha no Kyonyuu

You play Kazuki Seiya, who goes to a comic convention for the first time. Kazuki gets crashed on Amamiya Serika, a cosplayer with a perfect pair of big breasts. In spite of himself, he asks her to go steady, but her answer was given with a slap on his face.....

He tries to give her upm but few days later, he finds her at a book store. Without knowing she is seen by him, she did "it".......

[from English DLsite]