Kokushibo

Kokushibo (黒死牟) is Upper Rank One of the Twelve Kizuki in Demon Slayer: Kimetsu no Yaiba.

He is the strongest of Muzan's demons (excluding Muzan himself) and is widely regarded as the most powerful demon swordsman in the series.

Key characteristics:

• He is a tall, powerfully built man with long black hair tied into a ponytail, six glowing eyes (three on each side of his face), and prominent facial markings.
• He wears a modified kimono with the kanji for "Upper Rank One" engraved into his eyes.
• Kokushibo uses Moon Breathing (月の呼吸), a derivative combat style he personally created from the original Sun Breathing.
• His Nichirin-like blade is crimson and curved, capable of generating crescent-shaped attacks that are extremely fast and destructive.
• He has an exceptionally calm, stoic, and dignified personality, speaking in very formal language.

Major spoiler-related background (kept very light): Kokushibo is actually Michikatsu Tsugikuni, the older twin brother of Yoriichi Tsugikuni (the legendary Sun Breathing user), who turned into a demon out of jealousy and fear of mortality after centuries of life.

In short: the pinnacle of demon swordsmanship, tragic fallen samurai, and the deadliest blade among Muzan's Upper Moons.