Mobambi is a young human boy and the only human inhabitant of Crown Island, also known as O-Kan Island, a land populated entirely by strange and exotic animals. He has dark skin, red hair tied into a small ponytail at the top of his head, and a prominent scar across his chest. He wears a simple beige loincloth and often carries a locket containing a picture of his father.

Mobambi was originally the son of a zoologist. While traveling at sea with his father, they were attacked by the pirate-like hunter Count Butler and his crew. The attack killed his father and left Mobambi with the scar on his chest. He later drifted alone to Crown Island, where he was found and raised by the former animal king, a giraffe-like creature named Kirin Ryan. After Kirin Ryan grew old and died, Mobambi lived among the animals of the island, still grieving and nursing a deep distrust of pirates, whom he blamed for his father's death.

When the Straw Hat Pirates arrive on Crown Island following a volcanic eruption, Mobambi encounters the reindeer doctor Tony Tony Chopper. Because of an island legend foretelling that a new animal king would appear in a time of crisis, Mobambi immediately believes that Chopper is that destined successor. He enthusiastically hails Chopper as the new king and tries to convince him to stay and rule the island, initially showing hostility toward the other pirates in the crew due to his hatred of pirates in general.

Personality-wise, Mobambi is caring, loyal, and earnest. He genuinely worries about the safety of both the animals and Chopper, and he demonstrates a strong sense of responsibility for the island he considers home. He is initially fearful of direct conflict, a remnant of the trauma from his father's death, but he possesses a stubborn resolve to protect those he cares about. When Butler and his horn-hunting cronies begin attacking the island's creatures, Mobambi overcomes his fear. He retrieves Kirin Ryan's horn, the island's sacred treasure known as the "king's treasure," and attempts to lure Butler into a trap by throwing the horn into a volcano. The plan fails, and the treasure falls into Butler's hands, but Mobambi's courage does not waver.

Throughout the story, Mobambi's relationship with Chopper evolves from one of awe and projection to genuine friendship. He learns that not all pirates are evil, and he comes to understand Chopper's bond with his crewmates. He also forges a bond with the other Straw Hats, especially Luffy, who encourages him to stand up for what he believes in. His key relationship with his deceased father is symbolized by the locket he keeps, and his bond with the memory of Kirin Ryan motivates his actions to protect the island.

After Butler is defeated through the combined efforts of the Straw Hats and the island's animals, Mobambi initially feels guilt over having lost the sacred treasure. However, the animals and the Straw Hats recognize his bravery, selflessness, and growth. He is ultimately crowned the new animal king of Crown Island, fulfilling the island's tradition in his own right.

Mobambi possesses no supernatural abilities or combat training. His notable traits are his leadership potential, his intimate knowledge of the island's terrain and animal residents, and his growing courage. He relies on cleverness and determination rather than physical strength, and his most significant act is his willingness to risk himself to save others, which earns him the respect of the animals and the Straw Hat crew.