Background[]
Years prior to the start of the series, Momoki worked as a homicide detective in the 1st Investigative Division with Kokuryu Matsuoka and Akihito Narihisago, growing especially close with the latter and his family. However, on the 26th of October, 2016, Narihisago's daughter, Muku, was murdered by a serial killer, "the Challenger", with his wife, Ayako, going on to take her life. This would drive Narihisago mad with grief, going on to kill Denshin Katsuyama when he was revealed as the Challenger, ending up in prison while Momoki and Matsuoka continued to work as detectives.
Around two years later, after the invention of the Mizuhanome System and the formation of the Kura, Momoki and Matsuoka were recruited into the new organisation, with Momoki becoming the director of the Wellside, a special team that operated the Mizuhanome consisting of himself, Sarina Togo, Shirou Kokufu, Masamune Habutae, Sennosuke Shiratake and Kazuo Wakashika. Through this, he would also find himself working with Narihisago once again, who had been recruited as the Mizuhanome pilot, becoming Brilliant Detective Sakaido when inside an id well.
Some time after this, Momoki and Togo would start a relationship at least sexual in nature.