Kaigaku

After transforming into a demon, his worst human traits were amplified. He became sadistically cruel, taking genuine pleasure in mocking the pain of others, especially Zenitsu's pain and the tragedy he himself caused his former master. He shows not a shred of remorse for abandoning his humanity or for the lives he destroyed.

He is incapable of feeling gratitude. Kaigaku sees the world through a purely selfish lens: if someone helps him, he thinks they've only done their duty; if someone criticizes him, he assumes the person is envious or against him. He has never taken responsibility for his mistakes, always preferring to blame others or circumstances.

Kaigaku is extremely proud and demands to be recognized as superior. However, this pride hides a deep insecurity. He hates not being the center of attention or not being considered "number one".

Aversion to weakness: He deeply despised Zenitsu because he saw his companion as a weak crybaby, and he couldn't stand the fact that his master (Jigoro Kuwajima) treated them as equals.

Above any ideal, honor, or loyalty, Kaigaku's highest priority is his own survival. He has no shame in bowing down, begging, or betraying anyone if it ensures his continued life. For him, "winning" simply means surviving, no matter how low he has to stoop to achieve it.

He wields a modified Nichirin (hunter's sword). The blade features a pattern of black and yellow/gold lightning bolts, and the hilt has been altered to match his demonic powers and manipulation of dark electricity.

Demon Slayer Uniform: Curiously, even after allying himself with the demons, he retains a modified version of the standard black Demon Slayer uniform.

Haori: Over his uniform, he wears a dark blue haori (traditional Japanese coat) with black trim.

Accessories:

He wears a thick, light-colored cord tied around his waist as a belt.

Around her neck, she wears a necklace with various beads or small amulets in the shape of spheres (magatamas).

Expression: It generally displays an expression of arrogance, contempt, or constant anger. It also possesses quite sharp canine teeth.

Eyes: As a demon, his sclera (the white part of his eyes) become completely black. His irises are a light turquoise/blue-green hue. Engraved on his pupils is the kanji corresponding to his Upper Moon Six position (上弦六).

Hair: Short, spiky, and black, with a few strands falling lightly over the forehead.

Skin and Markings: His skin takes on a pale, grayish tone after the transformation. He has black markings in the shape of flames or lightning bolts that rise up his cheeks, extending to his neck and arms, a clear allusion to his Thunder Breathing technique.