Rando
Rando is a demon who killed 99 martial arts masters and stole their techniques. He sought to make it a tidy 100 by defeating Genkai, and entered the tournament she devised to determine her successor.
Although he disguised himself as an innocent young monk (the alias is known as Shaolin in the English manga and Shorin in the English anime), his true nature was revealed during his fight with Kazuma Kuwabara.