Saeyuku (冴え行く, Saeyuku; lit. "clearing gradually") is a former experiment of the Shinigami Research and Development Institute to mold the tasks of the Jigokuchō into a more sentient and intellectual being, although the attempt was deemed failed because of Saeyuku's muteness, which limited him from carrying out his task of transmitting orders and messages. He was locked up in a cell containing a spiritual suppressor, preventing him from escaping out of it ever again.