Momotaro is a flying squirrel who was transformed into an esper animal through military experiments, making him the sole surviving specimen of a wartime program to create animal psychics. He has yellow fur, black eyes, a light bulb on his back that he hides when in hibernation mode, and a crosshead screw embedded in his forehead as a result of the experiments. In the anime adaptation, Momotaro displays a cheeky, happy, and generally cheerful personality, whereas in the original manga he is depicted as less easygoing and more short-tempered. He serves as the constant companion of Kyousuke Hyoubu, often providing commentary on Kyousuke’s actions and plans, though he sometimes appears clueless about his partner’s true intentions.

Momotaro’s motivations are largely shaped by his traumatic past. He harbors a deep-seated hatred for humans due to the suffering inflicted on him during the experiments. This resentment occasionally surfaces in psychotic episodes when he is reminded of the experimentation, causing him to lash out with considerable force. Despite this, he develops a bond with the three young espers known as the Children, particularly Kaoru, who nurses him back to health after an accident caused by his stubborn obsession with sunflower seeds. Through this interaction, he begins to see Kaoru and her teammates as kindred spirits, even though his distrust of humanity remains unresolved.

In terms of role, Momotaro is a supporting character who accompanies Kyousuke in many of his activities and occasionally engages in combat. His notable abilities are sonic in nature: he can compress air around him into acoustic concussion missiles that he launches at opponents, and he can emit an intense, transient siren that scrambles and encrypts the brain waves of anyone nearby. His development is marked by a slow, tentative journey from a traumatized animal consumed by hatred toward a creature capable of forming fragile connections with those who show him kindness.