Muzan Kibutsuji

Muzan Kibutsuji (鬼舞辻 無惨, Kibutsuji Muzan) is the main antagonist of the series of Demon Slayer, he was the first and strongest demon that existed. He used his blood to create the other demons. He was able to manipulate his cellular structure and frequently disguised himself as different humans of various ages and genders. He transplanted his cells into demons to destroy them if they ever mentioned him by name. He could devour the cells of others to absorb their abilities and memories as well. Muzan was originally a sickly young man from the Heian period whose transition into a demon was the result of an experimental treatment to make his life longer. He killed his doctor before he even realized it worked. Since then, Muzan has dedicated himself to "completing his treatment" using demons to help him achieve his goal of having no weaknesses, with the sun being his only weak point. He also expressed justified concern towards those who used the "Solar Breathing" technique since his near-death experience in his fight against Yoriichi Tsugikuni. This motivated him to attack Tanjiro's family in an attempt to turn at least one of them into a demons, and after Nezuko, the sole survivor of the attack, manifests an immunity to sunlight, after he starts focusing his efforts on capturing and devouring her.