Mumei

Mumei (ムメイ, Mumei) is the last Twelve Shinshou member who was revealed. Throughout the series, he has been shrouded in mystery, as no one talked about him in any way.

Personality
Despite being a child, Mumei doesn't act as one and instead gives a sinister and eldritch vibe. Mumei appears having a behavior of an adult, having the knowledge and intelligence of understanding adult situations and even creating experiments on his own. But with the latter hobby, Mumei is malicious when it comes to human experiment such as resurrecting Munakata's deceased son and turning him into a killing machine who turns out to be the demonic Harunawa, and killing and experimenting countless lives, that even Kadowaki is appall by it.

Shimu
Unlike most of the Six Shinshou who may or may not be aware of Mumei, Shimu seems to be the only one who is close him. Mumei and Shimu are cooperating in terms of experimenting and plotting out their plans in first draft. Also, it may indicate that Mumei is heavily relying on Shimu when it comes to advanced technology since most of his experiment are done under Shimu's roof.

Trivia

 * It may speculate that Mumei's Hayagami powers are time considerate he displayed two time clocks on him.
 * It may most likely that Mumei was originally an adult as he may used time Kamui to reverse his age.