Jirō Tanaka works as an assistant for a science magazine publisher specializing in unexplained phenomena. Abandoned as an infant at Rock's coffee shop, he was raised there by Rock alongside Taro J. Suzuki, forming their non-traditional family. Though only grade-school age, Jirō exhibits surprising maturity and resents being treated like a child. He often spends time in the coffee shop below the publishing office, working alongside Taro and his dog Saburo. Jirō possesses latent healing abilities, which he initially keeps hidden.

Jirō is actually a reincarnated Motarian, an ancient being from the lost Mo Continent. He bears a concealed "お"-shaped marking enabling transformation using a MOPart artifact. Initially, these transformations trigger Alternate Identity Amnesia, leaving him unable to recall events in his powerful beast form. He gradually overcomes this limitation, gaining full control over his transformations and retaining memories in both states as he accepts his heritage.

His Motarian form grants significant combat prowess, allowing him to defeat Noin with a single strike, though he struggles against Catherine Sue, who nearly kills him. His connection to Saburo deepens upon discovering the dog is also a Motarian. Saburo's revelation of his true form as a massive bird and their shared history of reincarnation profoundly impacts Jirō's understanding of their bond and purpose.

Jirō helps thwart Catherine's plan to activate the doomsday device Ra Mo and eradicate humanity. During the climax, he uses his healing abilities to resuscitate Edward Chi after a near-fatal injury, demonstrating both his power and capacity for compassion despite their previous antagonism. Following the resolution of the Mo Continent crisis, Jirō remains with the magazine staff as they embark on new investigations into other ancient civilizations hinted at by discoveries from the Four Tune organization.