Gin
Ichimaru

Gin Ichimaru smiles constantly, keeps his eyes closed, and talks like a fox in a children's story, and the show uses him for a hundred episodes as shorthand for something being wrong. He is Aizen's right hand through the entire Soul Society deception, the captain of the 3rd Division who taunts Rukia about her execution, needles every hero he meets, and follows Aizen to Hueco Mundo without a word of explanation, abandoning his lieutenant Izuru Kira and his childhood friend Rangiku Matsumoto.

His Zanpakuto matches the persona: Shinso, a blade that extends at extreme speed, and in Bankai, Kamishini no Yari, which he cheerfully calls the fastest sword in existence while lying about how it works. In the fake Karakura Town he cuts Ichigo down almost lazily and asks Aizen why he keeps letting the boy live.

Then the mask comes off in a single scene. Alone with Aizen after the ascension toward the Royal Key, Gin puts his hand on Kyoka Suigetsu, uses the poison hidden inside his Bankai, and rips the Hogyoku out of Aizen's chest. The whole century of loyalty was a con: as a child he watched Aizen's underlings hurt Rangiku, and he decided the only way to kill Aizen was to get closer to him than anyone alive. It fails. The Hogyoku transforms Aizen and Gin dies in the rubble with Rangiku beside him, relieved that at the end he got to apologize. Izuru's line in the Blood War, describing himself as a dead man, is the show still turning over what Gin left behind.