Kaigaku

As a human, he's a muscular young man of average height with short, spiky black hair framing his face in two thick strands and a pair of dull, cold turquoise eyes framed by thick, arched eyebrows. He has an unfriendly face, permanently etched in a semi-permanent snarl.
After becoming a demon, Kaigaku gains many demonic traits: four black, flame-like marks on either side of his cheeks, elongated fangs, sickly pale skin, sharp, blackened nails, and pointed ears.

He usually wears a black kimono with a blue sash over his Demon Slayer uniform, which has yellow markings on it.
Proud - will serve anyone who recognizes his strength.
Smart - uses breathing techniques almost perfectly
He hates weak people - for example, when he was a student with Zenitsu, he saw him as a weak person, so he despised him and pretended that he didn’t like him.
He is known for his short temper - he is easily angered and lashes out at those who do so.
Capabilities
Enhanced Breathing Techniques - As a demon, Kaigaku's Thunder Breathing techniques have been strengthened and improved.
Electricity Manipulation Ability - Kaigaku uses electricity to enhance the power of his attacks, creating a sword effect that cracks and burns the opponent's flesh.
Swordsmanship - Different from most Demon Slayers in that Kaigaku wears his sword sheath on his back, rather than on either side of his waist.
He was originally a demon hunter and a member of the Demon Slayer Organization, and a student of Jigoro Kuwajima alongside Zenitsu. He is currently a member of the Twelve Demon Moons, holding the position of Upper Moon Six.

Now he is head over heels in love with Zenitsu, but he hides it as best he can.