Demon Slayer Chapter 177

Demon Slayer Chapter 177, titled "Younger Brother," delves into Kokushibo's past as Michikatsu Tsugikuni, revealing his deep-seated jealousy towards his twin brother, Yoriichi Tsugikuni, and the origins of his transformation into a demon. Kokushibo reflects on Yoriichi, believing anyone untouched by jealousy or who hasn't met someone like Yoriichi, favored by the gods and shining like the sun, is fortunate. Twins were considered an ill omen at their birth, causing a succession struggle. Yoriichi, born with a large mark on his forehead, was almost killed by their father, but their enraged mother intervened. It was decided Yoriichi would be sent to a temple at age ten to live a priestly life in the thrilling narrative of the Demon Slayer manga.

Their upbringings were starkly different: separate rooms, clothes, education, and food. Yoriichi was secluded in a small room. Michikatsu initially saw his twin as pitiful, believing Yoriichi clung to their mother due to poor treatment. Secretly, Michikatsu gave Yoriichi a handmade flute. Yoriichi never smiled, not even as an infant, and didn't speak until he was seven, leading many to believe he was deaf. Michikatsu first realized Yoriichi could speak when he was practicing swordsmanship. Yoriichi, standing silently in the shade, suddenly asked if Michikatsu dreamed of being the greatest samurai. Stunned by his unexpected voice and clarity, Michikatsu dropped his wooden sword. Yoriichi then proclaimed he would be the second greatest, deferring to his brother, as Michikatsu noted Yoriichi was soon to become a monk. Yoriichi then smiled, unnerving Michikatsu, who believed someone so attached to their mother could never be a samurai.

During their next training session, Yoriichi stood closer, hoping for instruction. A trainer, humoring him, gave him a bamboo sword. After demonstrating a proper stance, the trainer took his own. Instantly, Yoriichi lunged, landing three quick blows before a fourth knocked the man down. Michikatsu was astonished; a seven-year-old had landed four strikes instantly, leaving fist-sized welts. Yoriichi then declared he no longer wanted to be a samurai, uncomfortable with striking others. Michikatsu pressed him for his secret, and Yoriichi revealed he observed an opponent's skeletal alignment, muscular contractions, and blood flow. Michikatsu didn't understand until later: Yoriichi could see inside living things. The mark on his forehead granted him extraordinary vision and matching physical might. His once-pitiful brother now surpassed him in every way, yet Yoriichi preferred playing sugoroku and flying kites. Michikatsu, despite relentless training to master the sword, was a turtle compared to Yoriichi. He had been given the gifts, but Yoriichi pursued swordsmanship effortlessly. Soon, Michikatsu was confined to Yoriichi's old room as his father's trainer declared Yoriichi the successor, shattering Michikatsu's dream. One morning, Yoriichi informed him of their mother's death, apologizing briefly before departing for the temple. Yoriichi then showed Michikatsu the flute, stating he would cherish it, never losing heart, no matter their distance. Michikatsu, disgusted by Yoriichi's simple joy in a crude gift, couldn't reply. Yoriichi bowed and left with only a bag. Later, Michikatsu read his mother's diary, discovering Yoriichi had known of her illness for years and clung to her not from weakness, but to support her left side. This realization ignited Michikatsu's jealousy, leading him to tear the diary and curse his twin brother, marking the tragic descent of Michikatsu Tsugikuni into the envy that would eventually lead him to become Kokushibo, the Upper Rank One demon in the climactic Infinity Castle Arc of the Demon Slayer manga.