Gekkouin

Gekkōin 「月光韻, Moonlight Rhyme」 is a high-ranked manifestation-type demon of the Black Demon Series. It formed a contract with Yoichi Saotome, and became his Demon Weapon.

Appearance
Gekkōin manifests as a bright intangible and amorphous flame-like bird demon.

In Yoichi's mind, Gekkōin often appears as Yoichi's dead older sister.

While possessing Yoichi, Yoichi's body sprouts two short horns on top of his head, develops fangs, sharp nails, and has abnormal eyes.

Personality
Gekkōin hates weak-willed individuals and initially couldn't stand the pure Yoichi. Later on, they apparently become friends, as Yoichi states they chat in his dreams.

History
Gekkōin was a demon who was cursed within a bow.

Second Shibuya High Arc
When Yoichi attempts to make a contract with Gekkōin, Gekkōin takes over, and forces Yoichi to relive his sister's death on repeat. Gekkōin then breaks the illusion and complains about Yoichi's lack of greed or desire. Claiming that Yoichi is unfit to be his master, Gekkōin possesses Yoichi's body.

Gekkōin fights against Guren, Shinoa, Kimizuki, and Yūichirō. Guren orders the two boys to kill Yoichi, but Guren eventually manages to catch Yoichi's attention. Yoichi breaks free of the illusion and says he wants the power to keep his friends safe, thus recovering the control over his body.

Post-Shinjuku Arc
In Chapter 19, Yoichi manifests his demon and eliminates three horsemen at once. He has such a high aptitude for controlling gear that he did this on his own without any training or coaching. This is normally impossible. He reveals that he and Gekkōin chat in his dreams and even talk about their favorite colors.

Forms and Abilities

 * Demon's Thrall: Gekkōin may consume Yoichi with illusions in order to break his will and take over his body. This ability may only be used against the demon's host.
 * Basic Form: A big, black bow with a glowing eye-like parts at the upper and lower curves of the bow. It can manifest multiple arrows and can see far away. This allows Yoichi to target multiple enemies as well as spot them before they can attack. It is a purely long-range Cursed Gear weapon.
 * Miniature Form: A compact form of the weapon. It is easier to carry and to hide. A special ability of manifestation-type weapons.

Quotes

 * "Well? What now? Do you want power? I rifled through your heart, but I couldn't find any truly strong desire for revenge. All I found was stability. Kindness. Concern for others. I can't stand any of that. My sustenance is greed and desire."--Gekkōin to Yoichi Saotome
 * "Lies. You were saved... and that makes you glad. You don't like fighting. You hate the thought of friends or family dying. And you can't stand the thought of killing anything yourself. You want revenge, but you can't kill? C'mon. You don't have nearly enough greed. Wishy-washy humans like you... I hate. So I think I will become you and get revenge for your sister in your place. Give me your body, Human. You don't even have a strong enough desire to resist. No, you will not do at all. You are not fit to be my master."--Gekkōin to Yoichi Saotome
 * "Hmm... Dangerous humans. I'd better kill them all."--Gekkōin after possessing Yoichi