Funetaro Momoki began his career as a homicide detective in the 1st Investigative Division, partnering with Kokuryu Matsuoka and Akihito Narihisago. He grew close to Narihisago’s family, a bond shattered when the serial killer "the Challenger" murdered Narihisago’s daughter, Muku, prompting his wife’s suicide. Momoki persisted in detective work as Narihisago sought vengeance, killing the Challenger and landing in prison. Post-Mizuhanome System’s creation, Momoki and Matsuoka joined the clandestine Kura organization.

Leading Kura’s Wellside team, Momoki directed Mizuhanome operations with specialists Sarina Togo and Shirou Kokufu, coordinating with Narihisago—now the system’s star pilot, "Brilliant Detective Sakaido." Publicly, he spearheaded investigations into serial killers via their id wells; privately, he probed the mysterious disappearance of Kiki Asukai, the Challenger’s sole rescued victim. His relationship with Togo evolved into a romantic, at times sexual, entanglement.

Amid Kura’s operations, Momoki faced allegations of being the shadowy orchestrator John Walker, resulting in his arrest. The accusation hinged on John Walker’s exploitation of Kiki Asukai’s telepathy to manipulate killers, though Chief Takuhiko Hayaseura was ultimately exposed as the mastermind. Following the case’s resolution, Momoki resumed his pivotal role in Kura, as depicted in *ID:INVADED #BRAKE-BROKEN*.

In the sequel, he navigates emerging conspiracies and fresh investigations, maintaining a cautious partnership with Narihisago. Their dynamic blends professional reliance with Momoki’s insistence that Narihisago disentangle his psyche from the Sakaido persona, underscoring his dual focus on justice and the fragile humanity of those entangled in Kura’s labyrinthine pursuits.