Gekkouin's weapon takes the form of a big black bow with glowing eye-like parts at the upper and lower curves of the bow. He manifests as a black and green intangible and amorphous flame-like bird demon.
In Yoichi's mind, Gekkouin often appears as Yoichi's deceased older sister, Tomoe. While Gekkouin is possessing Yoichi, his body sprouts two short horns on top of his head, develops fangs, sharp nails, and has abnormal eyes.
In chapter 51, Gekkouin's real form is revealed. He appears as a well-built, young man with tousled light hair and a pair of curved horns sprouting from his head. His pointed ears and fangs reveals that he was once a vampire. He wears a long, black coat that has the front open to reveal his body, along with matching pants and boots. On his left arm, he dons a sort of spaulder that reaches down midway of his arm. Additionally, he also wears black, finger-less gloves. The left side of his face is covered with a black, curse-like mark.
Gekkouin 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.
Gekkouin's past is mostly unknown, but judging by Gekkouin's pointed ears and fangs, he was once a vampire before somehow becoming a demon. At some point, he was then sealed and cursed within a bow, and became part of the Black Demon Series.
Events of 2020
Second Shibuya High Arc
When Yoichi attempts to make a contract with Gekkouin, Gekkouin takes over, and forces Yoichi to relive his sister's death on repeat. Gekkouin then breaks the illusion and complains about Yoichi's lack of greed or desire. Claiming that Yoichi is unfit to be his master, Gekkouin 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.
After becoming Yoichi's gear, Gekkouin is summoned by him and eliminates three horsemen at once. Yoichi has a high aptitude for controlling cursed gear, that he did this on his own without any training or coaching. This is normally impossible.
Yoichi reveals that he and Gekkouin chat in his dreams, and even discuss about their favorite colors.
At Nagoya City Hall, Shinya and Yoichi prepare to snipe at Thirteenth Progenitor Crowley Eusford. Yoichi fires Gekkōin at him but Crowley catches the manifested Gekkōin with his hands. Unharmed, he then flings it aside.
Gekkouin reappears in chapter 51 in Yoichi's mind. Gekkouin reminds Yoichi of his sister’s death and asks him why he is not getting angry. Yoichi tells him to stop and to make the illusion go away. Gekkouin refuses, stating that Yoichi will forget about his revenge if he does that. Yoichi says he is not running away and points out that even Gekkouin cannot handle all of the high-ranking vampires.
When Gekkouin says he will get help from Yoichi’s buddies, Yoichi says he refuses to get them mixed up in his revenge. Gekkouin says Yu is using Yoichi for his problems and is taking advantage of them to get his family back, but Yoichi says that is completely different. He explains that Yu is a nice person who refuses to give up and keeps trying to get his family back and to protect his friends. He says that is truly amazing and insists that despair and hopelessness would have devoured him if he had not been lucky enough to become part of his family.
Gekkouin points out that Yoichi is implying he is not nice like Yu, and Yoichi agrees. He says he is so mean and nasty and petty that he cannot let the others find out. He explains to Gekkouin that they must make Lacus suffer and cannot kill him quickly or cleanly, but they cannot let his friends know. Gekkouin laughs and agrees to help him.
Forms and Abilities
- Demon's Thrall: Gekkouin can enter his host's memories and dreams to create nightmares of his host's past in order to break his host's will and take over their body . This ability may only be used against the demon's host.
- Basic Form: Gekkouin takes the form of a big black bow with glowing eye-like parts at the upper and lower curves of the bow . It is a purely long-range Cursed Gear.
- Miniature Form: A compact form of the weapon. It is easier to carry and to hide. A special ability of manifestation-type weapons.
- Arrow Manifestation and Sniping Capabilities: Gekkouin 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.
Gekkouin's contractor. Although Gekkouin initially wins possession of his body, Yoichi's feelings for his friends soon give him the willpower to regain control. That as well as wishing to do something to help them when he couldn't before for his sister. They later become friendly and chat in Yoichi's dreams , even going so far as to discuss their favorite colors. He has tremendous balance and potential.
- "Hmm... Dangerous humans. I'd better kill them all."--Gekkouin after possessing Yoichi, Chapter 6, "Black Asura"
- "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."--Gekkouin to Yoichi Saotome, Chapter 7, "New Family"
- "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."--Gekkouin to Yoichi Saotome, Chapter 7, "New Family"
- "Your sister was killed. Remember? Killed in cold blood by that Lacus vampire. He's standing right there, and you still aren't getting mad? Wuss."--Gekkouin to Yoichi Saotome, Chapter 51, "Crucifying the Immortal"
- "Really. Then, let me take over your body. I'll kill that blood-sucker for you. I'll kill him good."--Gekkouin to Yoichi Saotome, Chapter 51, "Crucifying the Immortal"
- "Yeah, but that Yu guy is totally using you for his problems, y'know. He seems a-ok with taking advantage of you guys to get his family back. Turn-about is fair play."--Gekkouin to Yoichi Saotome, Chapter 51, "Crucifying the Immortal"
- "Uh-huh. So you're implying you aren't nice like him?"--Gekkouin to Yoichi Saotome, Chapter 51, "Crucifying the Immortal"
- "You got it. See? This is why I like you, Yoichi."--Gekkouin to Yoichi Saotome after he reveals his intention to torture Lacus, Chapter 51, "Crucifying the Immortal"
- In the rough draft for Chapter 51, Gekkouin's real form was originally supposed to have long hair rather than short hair.