has had a rather varied group of monsters in its run..
- Most famous is the Elder Toguro. In contrast to his honorable Noble Demon younger brother, Toguro freely admits to being a treacherous sadist devoid of honor. In his first appearance, he kills Yukina's only friends, a pair of bluebirds, with great relish. In his first match at the Dark Tournament, he spares two fighters and promises to let one join Team Toguro. When one begs for his life, Toguro kills him...and then kills the other because he just hates keeping his promises. After the death of Genkai, Toguro uses his powers to make a mockery of her in front of everyone, particularly bringing it up to try to break Kuwabara's spirit during their fight, after he had already beaten and impaled Kuwabara repeatedly. His crowning moment of evil was letting the psychic Gourmet devour him... and then he tortuously devoured him from the inside out, delighting in his pain all the while. Later on he would use the Gourmet's powers to eat Yusuke's ally Murota alive. All of these crimes were committed while Elder Toguro was helping in two attempts to open a portal from the demon world into the human world which would lead to the humans being overrun and massacred by the demons.
- Gonzo Tarukane, the Smug Snake, Big Bad Wannabe of the Toguro arc is a Fat Bastard extraordinaire who cares nothing but for what will make him more money. To that end, he had Yukina, a gentle Ice Maiden captured and tortured to force her to cry, as her tears are precious jewels. He hires the Toguros to torture her to that end and is revealed to have physically tortured Yukina in the past. After this, he had one man who tried to help Yukina executed on the spot and taunted her about it. Vile, greedy, cruel and cowardly, Tarukane was the worst humanity had to offer in the series.
- Mad Scientist demon Dr. Ichigaki makes a kind martial arts master sick so his followers make a Deal with the Devil with Ichigaki to cure the master and save him by joining Ichigaki's experiments. He proceeds to turn said followers into bio-engineered Brainwashed and Crazy killing machines, and doesn't even cure their master. Ichigaki openly admits to not caring about said brainwashed killing machines and only entered into the tournament to get Yusuke's body; once that happened, he'd dispose of the disciples himself. Hee takes a sick pleasure in watching Yusuke and Kuwabara choose between winning the match (and killing the innocent followers) or trying to save them (and end up slain).
- Finally, there's the Makai ruler Mukuro's stepfather. He outfitted her with cybernetics to turn her into his sex-slave at BIRTH', and started raping her as soon as the procedure was complete. This continued throughout her childhood until she disfigured herself with acid so she'd be too unattractive for him to be interested, at which point he simply threw her out. Just to make sure she wouldn't try to kill him later on, he implanted Mukuro with false memories of him having been nice to her before. He had to implant these memories because he'd never shown her any kindness in her life.