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 dark 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 could not stand the pure hearted Yoichi. Someone having a gentle soul, being kind and having a great love for others are all qualities that he despises. 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 a demon, Gekkouin takes over and forces Yoichi to relive his sister's death at the hands of the vampire Lacus. Gekkouin assumes the form of Tomoe and opens her deceased eyes as he has Yoichi relive his most traumatic memory in detail. He has the visualisation of Tomoe pull Yoichi out form under the bed and Gekkouin says because of him she is dead now.As Tomoe he sits bored flicking a lamp switch on and off, then responds that Yoichi is the one who called him here and if he wants to avenge his sister's death he is going to need a strong set of cursed gear to wield. Regarding the form of Tomoe that he is taken and told it is pretty mean to look like that, Gekkouin replies to wait until Yoichi sees his true form. Turning to the matter at hand, Gekkouin feels there is a bit of a problem, Yoichi's desire is too weak and his soul is gentle and kind. Telling that Yoichi is lying about wanting to avenge his sister, Gekkouin says the strongest feeling he has is relief that he survived the attack. Further issues are that Yoichi would make a terrible fighter, is afraid of conflict and the thoughts of his friends and family getting hurt. Labelling him weak, without the courage to take a life, Gekkouin questions how Yoichi is to get revenge if he is not capable of killing. What Gekkouin really means is that Yoichi is a coward, just a child without a backbone. A black mist appears as Gekkouin explains one way to bring hell upon those who murdered his sister, by Yoichi giving him his body, forever. Tomoe's hand is used to stab into Yoichi and Gekkouin mocks the idea of Yoichi being a suitable master when he is the most pathetic human being he has ever met. Gekkouin is successful in possessing him and the Yoichi demon hybrid's first act is the gain some distance from the magic circle it was in. Within a ceremony room perched on top of a demon statue, the creature counts four little humans all for it. Damning it since they each have cursed gears too, it remarks how power hungry humans are such a pain then calls for its bow. They will be its appetisers, then the real feast will begin. Firing and leaping across the room, the demon uses the physical bow to fend off Kimizuki's attack before kicking him. With the bow knocked out its hand by Yu, it then grabs Yu to throw him to the ground. Placing its hands up with Kimizuki's sword near it neck, the demon speaks to them that it is interesting they will not touch him because of the body it is in. Continuing to try and attack, the demon has regained its bow and tells Yu it is too late and to die. Faltering when Yu says he will not abandon a comrade, the demon begins laughing to itself. Appearing pleased when Yu casts his weapon aside, the demon calls him a fool as it takes aim. 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 who was able to learn his name and access his most traumatic memories. Appearing bored at Yoichi on account of his kind nature, as Tomoe he takes ahold of his face and calmly explains the problem. 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 could not 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.