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The protagonist, Nakaido Akira, is a bit of a delinquent. He is on the roof playing instruments with other students when his classmate, Suzuna Aya, comes to fetch him. She's pretty short tempered, so Akira escapes to skip school and go home.
On the way home, he laments on how boring his life is. His thoughts are interrupted by a tall dark stranger, however. To Akira's surprise, the man knows his name. Introducing himself as Shirogane, the man asks Akira to come with him. A confused Akira refuses, saying it would be stupid to go with a suspicious person. Shirogane says that he is in fact, not a person at all, and reinforces that by showing Akira that he has no shadow. Shirogane then disappears.

Raping time.

At home, Akira vows to stay inside, but his friend Asamura Kengo drags him out. They go motorcycle riding, when Kengo loses control and crashes into the back of a truck.

Akira and Kengo have just finished playing a motorcycle racing game at the arcade when they see a young boy being robbed by some thugs. The boy's guardian, an elderly gentlemen, is thrown aside by one of the thugs. Akira saves the two, and they find out that the boy's name is Haruka. Haruka has good enough grades to get into an American university, despite having the appearance of an elementary school student.

Aya appears, chasing Akira and Kengo from the arcade. Once she corners them, they prepare for a beating, but she instead asks them to escort her to school.

tsun tsun - dere dere

She's left her precious dragon doll at school, and can't sleep without it. (Why would you bring a stuffed animal to school? I mean, I can identify with not being able to sleep withou- forget I said anything.) With the doll safely retrieved, Aya contends that the school is scary at night, so Akira teases her for being scared of ghosts.

The group is suddenly assaulted by a formless black monster. They run, but Aya beats Kengo senseless from fear, before fainting herself. Cornered, Akira takes Aya's shinai and fends off some of the monsters. However, he's disarmed, and would be overwhelmed if Shirogane had not appeared.

Shirogane erects a forcefield around them, and tells Akira that the monsters are called 'Kokuchi' that come from the shadow realm world. He wants Akira to bind a contract with him, which Akira refuses because he is homophobic still doesn't trust Shirogane.

Shirogane just wants some hot sexxor. What's not to understand?

Despite Shirogane's warnings, Akira breaks from the forcefield and runs for it. He doesn't get far, at the school grounds the concrete begins sucking him in. Immersed apparently underwater, Akira seems to 'lose' a part of himself. Shirogane pulls him out, and says that Akira lost his doppelganger. Apparently, if one's doppelganger ceases to exist, so does oneself.
Under the threat of dying, Akira decides that BL is better, and agrees to Shirogane's contract. After pulling some fancy magic stuff, Shirogane kisses Akira (finally!), changing Akira's eye and hair colour.
Transformation complete, Akira discovers that he's gained a significant stat boost, and is now an assassin-class. Akira and Shirogane then proceed to dispatch a good number of kokuchi.

キタ━━━(゜∀゜)━━━!!!!!

However, after the battle Shirogane tells Akira that he's no longer human, and points out Akira's lack of a shadow. Shirogane tells Akira that he cannot return human unless he kisses Shirogane again. Akira is taken aback, but Shirogane disappears and Akira returns to normal. His friends Aya and Kengo come running, relieved that he's okay. He contends that maybe being bored is not so bad after all.

Thoughts:
Very gay. Probably won't win against Junjou Romantica, but it's enough so that I could call it the poor man's Junjou. Anyway.
Plot seems rather typical, mysterious mystical figure approaches normal high-school kid with the intent to sign a contract since the latter apparently has some sort of special mystical blood-line/chance/spiritual power and is the only one the mystical figure can fuse with. One of the sides also harbors feelings for the other, except, you know, usually these pairings are of the opposite sex.
RIGHT so since I can't get my mind off of the gay, I think just stop trying so hard. Let's see… um, Aya probably likes Akira and the kokuchi were probably sent by some big bad evil overlord (or Haruka) to sink the world into dark chaos etc etc. ksleeptimebye