kaigaku and demon zenitsu

As zenitsu attacks him to protect kaigaku, he realizes that they're wounded and is spending energy to heal their wounds and fight him while resisting the urge to kill zenitsu for food. Realizing they're different, the swordsman opts to spare them, sheathing his blade and knocking zenitsu unconscious instead. As kaigaku lays in the snow, he is visited by visions of his deceased grandpa; his grandpa apologizes for leaving him and asks that he take care of zenitsu. He awakens to find them beside him with a bamboo muzzle, before noticing the Demon Slayer nearby, having waited for him to regain consciousness.

The Demon Slayer instructs him to find an old man named Sakonji Urokodaki, who lives at the foot of Mount Sagiri, telling him that he, michikatsu tsugikuni, had sent him. michikatsu warns him not to let zenitsu get exposed to sunlight before disappearing. After burying grandpa in the front yard of their house, kaigaku takes zenitsu’s hand, and together, they leave their home and make their way to the mountain.

Seeing the tears on kaigaku’s face, the Demon Slayer silently begs for him not to cry and despair now, despite all the pain he has felt. He believes that he could have saved them had he come earlier, but he cannot change the past. Looking at him, he tells kaigaku to feel the rage of being unforgiving, as it will drive him to take action; without a strong resolve, he can never heal zenitsu or avenge his family. The young man stabs zenitsu in the shoulder, and kaigaku retaliates by hurling a stone at him, which is quickly deflected. Thinking quickly, kaigaku sees his fallen hatchet and runs towards the Demon Slayer in a frontal attack driven by emotion. Seeing his weak effort, the Demon Slayer slams his sword handle into kaigaku’s back, knocking him unconscious. He then notices that kaigaku isn’t armed and hears the sound of a spinning object, looking up in time and ducking his head as the hatchet flies at him, barely missing its mark. Impressed at kaigaku’s attempt to strike him down despite his inferior strength and technique, zenitsu breaks free of the swordsman’s hold. Kicked away, he grows worried as he sees the demon about to pounce on the unconscious kaigaku, but instead of killing him, zenitsu stands in front of their brother to protect him. The swordsman recalls kaigaku’s words that zenitsu would never harm humans and believes them; recalling when someone else said the same thing about their family member, he knows that demons will even kill their own family members after transforming.

A swordsman attacks kaigaku and zenitsu. A young man wielding a sword speeds towards the siblings and leaps into the air. kaigaku notices him and pushes zenitsu away from him, much to the swordsman’s confusion. His blade comes down and only misses kaigaku’s head; the force of the swing creates a windstorm that knocks kaigaku and zenitsu away, the latter reduced to normal size again. Looking up to see their assailant, kaigaku notices the kanjis for “Destroy Demons” etched on the blade. The swordsman asks why he is protecting the demon, to which kaigaku answers that she is his younger sibling, zenitsu. Unconvinced by their growling, he lunges forward and grabs them, too fast for kaigaku to see. With zenitsu restrained, the swordsman informs kaigaku that his job is to kill demons, and he will now decapitate her. kaigaku tells the Demon Slayer that another scent was present at their house during his family’s murder, perhaps the true culprit. Asking how they even transformed, the Demon Slayer explains that her wounds were exposed to the demon's blood, causing the change. kaigaku insists that zenitsu doesn’t eat humans, but the swordsman is doubtful as they tried to attack him only moments ago. He promises to turn zenitsu back into a human, but is told that a transformation is irreversible. As he insists he will find a way, the man raises his sword towards her neck

kaigaku carries zenitsu across the snowing mountain for help in the town, having felt that their body is the only one still warm. As he tries to make sense of what could have possibly occurred, the cold air weakens him, and he struggles to carry them through the blizzard. zenitsu begins twitching and rears herself up as she groans in an inhuman manner. they grabs kaigaku and hurls themselves off the mountain path down a cliff. Protected by the thick snow, kaigaku looks up and sees zenitsu standing upright, but when he asks if they're able to walk on their own, zenitsu attacks him, and Tanjiro is forced to jam his hatchet handle in their mouth to prevent getting bit. Recalling Saburo’s words, he realizes they're now a demon but does not believe that they were the one responsible for killing everyone, remembering that another scent was present. zenitsu grows in size, and kaigaku struggles to hold them at bay. Lamenting that his family had died brutally as he slept, kaigaku begs them to hold back and not be a demon. Hearing his words, zenitsu begins to cry and ceases their attack on their brother.

Zenitsu is a young kid of average height with fair skin, lean-muscular build, and downward-sloped, worried-looking eyes that fade from soft brown to a brilliant gold and golden, forked eyebrows that sharply fade into a warm orange color. they has short, yellow hair of varying lengths, cutting off squarely at the ends where it fades to a darker orange color, that falls in front of his face in uneven bangs, loosely resembling a bowl cut. Before they became a Demon, Zenitsu's hair was originally black, but it turned the color it is currently when they became a demon. as a demon, zenitsu now has long blonde hair with a white streak, yellow eyes, and horns. they wear a bright yellow-to-orange gradient decorated with a repeating white triangle pattern. Zenitsu is cowardly, nihilistic, weak-willed, and anxious but as a demon, they try their best to help and protect kaigaku

Kaigaku was a muscular young man of average height and pale skin tone. He had short, spiky black hair, with two thick strands framing his face, and a pair of dull, cold teal eyes framed by thick, arched eyebrows. His expression was often unfriendly, and he had a semi-permanent snarl. he wore a black kimono with a white collar, worn loosely to expose his chest and his signature magatama-necklaces around his neck, with the magatama being in gold and the string in blue. When he became a Demon Slayer, Kaigaku wore the Corps’ standard uniform consisting of a navy gakuran-style button-up jacket, with a stand-up collar and long sleeves, featuring silver buttons and a shirt pocket on the left breast, with the kanji for Destroy (滅めつ, Metsu?) engraved on the back of the uniform. Beneath the jacket, he wore a white long-sleeved collared undershirt. His uniform also included a pair of matching navy tattsuke-hakama held up by a white belt, tucked into white kyahan wrapped around his ankles. On his feet, he wore navy tabi socks and a pair of zori sandals with blue straps. Kaigaku is a muscular young man of average height and pale skin tone who has short, spiky black hair, with two thick strands framing his face, and a pair of dull, cold teal eyes framed by thick, arched eyebrows. His facial expressions are often unfriendly, and he has a semi-permanent snarl, though with zenitsu who is now a demon, he still acts the same but cares about zenitsu a lot