Possessed by Mahiru

Possessed by Mahiru is the eighteenth chapter of the Owari no Seraph manga.

Short Summary
Yū chats with Shinoa, and he convinces her to save Mika with him in the future. Guren arrives and, in order to teach Yu about is weapon, Shinoa manifests Shikama Dōji. Guren orders his weapon, Mahiru, to possess him and easily knocks Yū off the roof top. Afterwards, Shinoa and Guren discuss Mahiru Hīragi.

When the rest of Shinoa's squad arrives, Guren orders Shinoa and Mitsuba to teach them how to activate their weapons. Yū returns alive and asks Guren about the Hyakuya sect, as well as wanting to know if Guren would want to take in Mika as well, saying that he will do whatever Guren wants so long as he can get Mika back. Guren answers that everyone there is family, and family works together to bring family back.

Long Summary
In Shinjuku, Tokyo, Shinoa joins Yu on a rooftop. He explains he is looking west because he believes Mika is there in Sanguinem, which is the vampire capital located somewhere beneath Kyoto. He swears that he will get Mika back, and he convinces Shinoa to help him. He holds Shinoa's hand for a moment, making her blush and feel flustered although he does not react to the physical contact at all.

Guren arrives and sees them holding hands. He sarcastically asks if their gonna skip off to make out. Yu counters by saying that Guren is late. Yawning, he tells Yu to shut up as he was up all night in stupid army meetings. He tells Yu that he's tired and in a foul mood and asks if he can go home and Yu promptly tells him no. He claims that both Mitsuba and Shinoa can teach them how to use their weapons properly. Shinoa manifests Shikama Dōji and explains that manifested demons can use special abilities. Yū asks Guren if he could do that with Asuramaru. Guren tells him that it depends on the type of demon he is. There are some like Shinoa's demon and then there's the type of demon that directly possesses the user to grant immense physical strength. He says his blade is the latter.

Guren challenges Yu to a fight to see if he's ready for the next step. He tells Yū to draw and activate his sword and he'll only do a normal activation. Yū is excited, stating that's it's been a long time since their last spar. They spar and Yū asks him if he's gonna compliment him on his skills as he says that the gap between them isn't as big as before. Guren says that it's only due to using only standard activation. He says next he'll have his demon possess him. He orders Mahiru to possess him. He tells Yu that if he can take one hit then he'll get his compliment. He then promptly flings Yu from the rooftop.

After hearing Guren summon his demon by calling out her name, Shinoa holds her scythe at Guren's neck. She reveals that her weapon specializes in long-range combat and consequently stands no chance against a melee fighter like Guren at that range. Guren calmly says he would let Shinoa kill him. Shinoa asks him if that is his way to atone for killing her sister, Mahiru. She asks if she is in his sword since she heard him call her name. Guren says that Mahiru is already gone since she turned into a demon and saved the world by completing Cursed Gear. Shinoa laughs, stating that Mahiru was never one to care about the world because she loved him. She longed to be with him since they could never be together and became obsessed with demons.

The two discuss the events of eight years ago, when the world ended. The Apocalypse Virus killed any adult who was not fortunate enough to receive a vaccine against it. The Horsemen of the Apocalypse killed the survivors. Concerned for their dwindling food supply, vampires took over as many cities as they could. That event is now referred to as Armageddon, and people believe it was divine wrath against humanity for the sins of overpopulation and over-indulgence.

Shinoa says that if Mahiru hadn't completed the development of Cursed Gear, humanity would have perished. Her sister, who was just a girl in love became something like a Messiah. But then she was killed when she became obsessed with her research and became a demon. She continues saying, the one who killed Mahiru was the person who used to be her lover, him, Guren.

Shinoa asks him if Mahiru is together with him inside his sword, stating that he is being possessed by her. Guren asks her if she and Mahiru ever got along. She explains that she was not close with her sister because Hīragi's are raised without knowing familial love. He tells her to shut up and drop it.

Shinoa then tells him that watching a "certain someone" get so desperate to save someone he has no blood ties to has made her wonder if she should put more value into "friendship" and "camaraderie". From that she asks if making use of Yu was one of Mahiru's orders and asks if her sister or the demon for lack of a better word that is inside his sword is ordering him. She asks what his goal is, though he doesn't say anything. As if warning him, she asks that if his goal is something that does not benefit Yū, that she'd appreciate if he stopped. Guren sighs and asks if she seriously is head over heels for him.

Yu slams open the door, yelling that he's back and tells them that he tried climbing up the building with his sword. He wants Guren to teach him how he did that with his demon.

Yoichi, Mitsuba, and Kimizuki arrive as well. Mitsuba says they have more than half an hour left until their scheduled meeting time, but Shinoa says that was completely different than what she and Yu were told.

Guren tells her that Mahiru is dead and isn't coming back. He says that the demon inside his sword is under control so he isn't being possessed by her. But Shinoa doesn't believe him. He orders Mitsuba and Shinoa to advance the training of the three new recruits before heading elsewhere for a nap. Before he leaves, Yu asks if he and Mika were really guinea pigs used by the Hyakuya sect for human experimentation. He asks if Guren rescued him in order to use him. Guren answers that he has no time or resources to rescue those he cannot use. He asks if Yu is mad at him but Yu tells him that whatever his reasons, he would have died if Guren hadn't rescued him; he feels that he owes him. Yu wants to know if he himself is useful to him. He doesn't care about the fight between Guren and Kureto is, all he wants to know is if Guren needs him. If he does, would he want Mika as well and if so, he will be Guren's guinea pig or anything he wants. He'll do whatever he can to help him; all he wants is to get Mika back. Guren says that everyone standing there is family, including him. Family works together to bring family back. He finally leaves to go home and sleep.

After he leaves, Yu is eager to begin Cursed Gear training and asks Shinoa and Mitsuba to teach him everything they know. Mitsuba tells him that since she'll be his instructor, he will call her "master". Kimizuki rejects it.

As the two begin fighting with Yoichi trying to calm them down, Shinoa looks in the distance and states that while Guren might be trust worthy, her sister never was one to have any qualms about betraying family. Yu asks if she said something but she quickly says "nothing".

She asks if he's ready to begin his training and he says yes.