Chopper grew up on the winter island Drum, where he was an outcast among reindeer due to his blue nose. He ate the Hito Hito no Mi, giving him the ability to speak, think, and transform into a human. Chopper is very naïve, and has a tendency to believe anything told to him, even the outrageously fictitious, exaggerated lies of Usopp. According to Usopp, Chopper is the most gullible Straw Hat.
Chopper's left antler has a metal plate wrapped around it at all times. He originally broke it while retrieving a mushroom for Doctor Hiruluk. The name "Tony Tony Chopper" is partly a play on "Tonakai" ("Reindeer"), and partly on the idea that his horns look like they could chop down trees, according to Hiruluk
As the doctor of the crew, Chopper has impressive knowledge of medicine and medical drugs, as well as the preparation of remedies and ointments from regular fruits, roots and vegetables found on most islands. Most of this he learned during his apprenticeship under Dr. Hiruluk and Dr. Kureha.
Chopper is also capable of conversing with other animals.