At the time of his debut during the Final Selection, Genya was a short, young man with large, inward-slanting eyes. The space where his eyebrows would've been is constantly furrowed, which gave him a permanently angered appearance. His hair is styled into a buzz cut, with only a small clump of messy black hair left to grow on the top of his head. He also possessed a large, jagged scar that ran over his right cheekbone from his ear and to over his nose.
During his first appearance, Genya is seen wearing a purple, sleeveless yukata over a plain black shirt with long sleeves.
Later on, when seen again in the Butterfly Estate, Genya had a much more feral appearance, having undergone a growth spurt that put him at almost six feet tall. His hair is also much longer, worn in more of a mohawk style, the strands much longer and visible around his neck. It is also noted that he became more muscular and gained many more scars all over his body.
After becoming a Demon Slayer, his outfit changed to a dark olive-green version of the Demon Slayer uniform, over which he still sported his old purple yukata.
Genya is a foul-mouthed and ill-tempered individual; a "lone wolf" who refused any help from others and did not show any interest in being polite or having any manners. During the Final Selection, he roughly handled Kanata Ubuyashiki by grabbing her hair and yelling at her despite her being a child. His ill temper and brash way of speaking heavily resembles that of his older brother, Sanemi.
He became more open to cooperation during the battle against Hantengu, an Upper Moon Demon, where he worked alongside Tanjiro Kamado, Mitsuri Kanroji, and Nezuko Kamado. After some time, he learned to become more sociable and less ill-mannered towards others, particularly towards Tanjiro and his friends.
Sanemi's refusal to acknowledge Genya also made him insecure as he came to realize just how weak he is compared to his companions. Nonetheless, he did whatever he could to help his allies, even if it got him horribly maimed in the process. Despite his brother's harsh rebukes, he bore no ill will towards him, a belief reinforced by Tanjiro and eventually by Sanemi himself.
Abilities and Powers
Demonic Transformation: By consuming part of a Demon, Genya could temporarily gain demonic attributes from it. Depending on the strength of the individual, he could gain regeneration and physical strength equal to one. In this form, Genya is capable of overpowering Aizetsu briefly.
- Upper Moon Four Empowerment: After consuming the wood created by Zohakuten, Genya had enough strength and speed to throw 3 trees in quick succession. In his battle against Kokushibo, the full extent of his demonification upon eating some of the Upper Moon Four's flesh is shown that despite Kokushibo viciously bifurcating him across his abdomen and cutting of both of his arms, he still did not die, showing that like a genuine demon, Genya could now only be killed by decapitation.
- Upper Moon One Empowerment: Upon eating the hair of Kokushibo, Genya's demonic abilities were further enhanced, as seen when he managed to reattach his severed body in mere seconds and is even able to intercept the telepathic communication of Muzan Kibutsuji. Upon eating a piece of the Upper Moon's Flesh Sword, Genya even began taking up several of Upper Moon One's physical characteristics such as his eyes and hair. Later, the full extent of his enhanced demon abilities is shown that even when he is bifurcated and half his head is sliced cleanly in half, Genya is not only still alive and conscious but still possessed the necessary strength to activate his Blood Demon Arts. However, upon succumbing to his severe wounds, Genya much like a genuine demon, disintegrated to ash.
- Flesh Gun: Genya displayed the ability to detach his cells and merge them with his gun, with the gun having the same characteristics of Kokushibo's sword of having multiple eyes on top of the barrel and muzzle, which presumably increases its accuracy and power.
- Demon Slayer Mark: Genya also gained access to his Demon Slayer Mark that appeared as a flame-like tattoo design on top of the right side of his forehead which presumably also increased all of his physical and mental abilities.
Blood Demon Art (
- Wooden Flesh Bullets - Due to eating the cells of Upper Moon One and Upper Moon Four, Genya had developed a unique technique where he merges his demon-transformed cells with his sun steel bullets and gun, that, when fired, the direction and velocity of the bullets could be freely manipulated by Genya to easily move around obstructions between the bullet and the intended target, as seen when his bullets moved around Kokushibo's sword, directly burying themselves into his flesh. Once inside its target, the bullets can immediately grow into a gigantic tree similar to the tree creatures of Upper Moon Four's Zou Hakuten form and then entangle and wrap around the intended target, paralyzing them.
- Blood Roots - Genya could further alter his embedded Flesh Bullets by having them take root inside his target and suck the demons blood from inside their bodies, effectively preventing them from activating their own Blood Demon Arts.
- Genya's surname contains the kanji for "immortality" (不死 shinazu?) and "river" (川 gawa?).
- Genya is one of the few characters in the series who does not primarily use a sword and is the only Demon Slayer named so far who cannot use any of the Breaths.
- Generally, Genya is better at wielding a gun than a sword, as he's very good at hitting targets.
- It seems that Genya is shy around women, breaking out into a blush and losing the capability to speak with them. This is seen when interacting with the Love Hashira, Mitsuri, and the younger girls who work at the Butterfly Estate.
- Due to his unique Demon-eating powers, Genya regularly goes to the Butterfly Estate for health check-ups.
- Genya's astrological zodiac sign is a Capricorn.
- (To Sanemi Shinazugawa) "I don't want... you to die. Because... my big brother...is... the nicest person...in...this world." 
- Kimetsu no Yaiba Manga: Chapter 101, Page 11
- Kimetsu no Yaiba Databook
- Kimetsu no Yaiba Manga: Chapter 179, Page 17