Naraku

The main antagonist, Naraku (literally means "underworld", or "hell") is responsible for nearly every character's suffering throughout the series. He is a han'you that originated from a bandit named Onigumo ("Demon Spider"). Naraku's main goal is to collect all of the shards of the Shikon no Tama in order to become a full Youkai. His platonic love for Kikyo is what drove him to kill Kikyo and seal Inuyasha on the tree and the basic reason he became a demon.