She's the main character's "weird" neighbour whom he keeps a big secret that can't be revealed to any of his school friends. It's because of her that his life has turned topsy-tury, and it's from that secret that the story begins...

Kido Tatsuhiko moved to Tokyo to attend an art school and start his new life. In his new room, there's a small hole in the wall. At first he can see nothing through the small hole, but one night, through the peeping hole, he saw a girl named Ikuno Emiru, a perverted voyeur who ropes him into her peeping fetish... But each winds up getting more than they bargained for. 
Bundled with the limited edition of the manga's 13th and final volume. 
(Source: AniDB)

Tatsuhiko Kido, a vocational/technical school student who's staying in his school's dorms, one day discovers a hole in the wall of his apartment room. He peeks through the hole, but when he does so, he sees his neighbor, Ikuno Emiru, masturbating and he's caught peeking! However, when he goes over to inform her of the hole in the wall, she shuts the door in his face. Luckily for him, he's able to stop the door from closing and pursues after her, but, in his haste, he trips and lands right on top of her. As luck would have it, she takes a picture of them in a compromising position and tries to blackmail him into getting naked?!

Emiru Ikuno