Genya shinazugawa

At the time of his debut during 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. Genya is seen wearing a purple, sleeveless yukata over a plain black shirt with long sleeves.

Later on, when seen again in the Butterfly Mansion, 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 his boyish features got toned down, and he became more muscular and gained many more scars all over his body. His outfit changed to a dark olive-green version of the Demon Slayer uniform, over which he still sported his old purple yukata.

After consuming a piece of Aizetsu's flesh, Genya gained many physical qualities reminiscent of a demon. The tips of his hair gained yellow highlights, his mouth grew fangs, and his eyes became a demonic combination of black sclera and yellow irises. Genya was initially 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 Final Selection, he roughly handled Kanata Ubuyashiki by grabbing her hair and yelling at her due to his impatience, and he later provoked Tanjiro Kamado into directly confronting him with physical force as an attempt to diffuse the situation. 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, the Upper Rank Four, where he worked alongside Tanjiro, 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. Genya also appears to be very confident in battle, telling Aizetsu to focus on him despite the latter already having stabbed Genya with his Yari, and later announced his name to the demon and declared that he is the one who will kill him. As a child, Genya was caring and loving towards his family, looking out for them due to their father's abuse and being the second oldest of his siblings. His protective attitude stems from a promise he had made with Sanemi following their father's death, when he stated that it was now their job to be a role model for their siblings and look after their mother. He was deeply regretful of his inability to save his brothers and sisters following their mother's transformation and blamed himself heavily for his own accusation of Sanemi killing her, despite acknowledging later that he was not at fault and she transformed into an uncontrollable demon.

During Sekido's near fatal attack on Genya, he shed tears of regret when he believed he was about to die without ever getting the chance to properly apologize to Sanemi for his harsh words, only encouraged to push past it when Tanjiro had rescued him and placed his faith onto him. Sanemi's refusal to acknowledge Genya 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, such as shielding Tanjiro from Aizetsu's Blood Demon Art, and playing a pivotal role in the battle against Kokushibo. Despite his brother's harsh rebukes, he bore no ill will towards him, a belief reinforced by Tanjiro and eventually by Sanemi himself.