Mercy Kill: He shot 119 of his own men that were slowly dying from a gas attack. He keeps a rose garden containing 119 of them.
Villain Decay: Built up as the big bad of the Cape Arc by being the leader of the cape and easily defeating Riki-Oh in a fight...only to later go crazy, paint his face, wear weird head gear, and is suddenly much easier for Riki to beat.
Justified, as he was completely focused and composed in the first fight with Riki-Oh. In their second duel, he's gone insane, and Riki-Oh's been honing his skills in battles prior to this one.
Asshole Victim: He gets tortured by Aneyama for failing to kill Riki-Oh. Considering that this happens after we learn about his more unsavory aspects, it's hard to feel sorry for him.
Cold-Blooded Torture: He's implied to indulge in this, as he threatens Riki-Oh with brutal dismemberment before death, and delights in the idea of Riki-Oh being force-fed sulfuric acid.
Faux Affably Evil: He's introduced as a rather pragmatic, laid-back gangster who took the daughter of one of his underlings under his wing after Riki-Oh killed said underling in a duel. Before the arc is finished, however, this relatively nonchalant facade is revealed to be masking some ratherunpleasant habits...
Killed Mid-Sentence: He's slain by one of his own lieutenants just as he's about to give up the same of Riki-Oh's true father.
Non-Action Big Bad: Despite trying to blow Riki-Oh up with a rocket launcher in his introduction, he's ultimately helpless when Riki-Oh confronts him after killing off his lieutenants.
No Kill Like Overkill: When he first shows up to kill Riki-Oh, he brings a damn rocket launcher to do the job. Of course, considering that Riki-Oh survived being shot five times when he tried killing him in the backstory, this might count as being Dangerously Genre Savvy.
Even Evil Has Standards: Even when he was a full-on bad guy, he couldn't work directly under Mizuguchi after the crime lord orchestrated the mass murder of orphans and the homeless with poisoned food.
Heel-Face Turn: Introduced as an Elite Mook for a low-level gangster, but chooses to follow Riki-Oh for the Mizuguchi arc.
Heroic Sacrifice: When both he and Riki-Oh are mortally wounded by one of Mizuguchi's lieutenants, he chooses to give Riki-Oh a crude form of blood transfusion so that he might live...at the cost of his own life.
Cold-Blooded Torture: He takes sadistic delight in tormenting his victims—in the backstory, he tortured Riki-Oh's adoptive father and grandfather; in the story proper, he punishes Mizuguchi rather savagely for failing to kill Riki-Oh in the backstory. Long story short, it involves a power drill, a hollow tube of bamboo, and boiling soy sauce.
Creepy Crossdresser: Downplayed at first, with his penchant for nail polish and lip gloss. However, in the climax of his arc, he goes to wearing a full-out bridal kimono in honor of his deceased sister.
Berserk Button: Don't do anything uncouth in his presence, like leave a hair in his tea. Or mention his true age.
Depraved Homosexual: He has a creepy fascination with Riki-Oh at first. Later, this is lost beneath a hateful desire to see Riki-Oh dead.
Disproportionate Retribution: His butler leaves a hair in his tea. Aneyama's response is to butcher him with a razor-edged ceiling fan. Later, when another of his servant's accidentally makes a remark about Aneyama's true age, the enraged villain kills him with another remote-controlled death trap.
Powered Armor: Used one during his fight with Riki-Oh to boost his strength. It looks suspiciously like RoboCop's armor.
Villainous Breakdown: He begins to steadily lose it when Riki-Oh begins tearing apart his operation. At the very end, he completely loses his head when his cosmetic surgery is destroyed, leaving him as an ugly old man, and throws himself off a cliff to his death.
Disney Death: He's on death's door when Aneyama's "Crystal Night" plan goes into effect. Riki-Oh is guided by Nachi's spirit to use the power of his six-point-star hand to save the kid's life.
Tag Along Kid: Notable, because he's the last of Riki-Oh's companions who follows him for the remainder of the story.
Ace Pilot: He's able to fly his plane through regions that would cause lesser pilots to crash. This makes him a key ally to Riki-Oh, as Mukai's domain is in a part of the Arctic Circle that screws up the instruments of most planes.