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 (ヒトヒトの実, Human Human Fruit), giving him the ability to speak, think, and transform into a human. This ability further alienated him from the other reindeer, and he was rejected by the herd. He grew up an outcast until taken in by the fake doctor Hiruluk, a former thief, who accepts Chopper as his assistant and instills within him his initial passion for medicine.
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, which is probably the most truthful thing he's ever said.
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, the fake doctor who was Chopper's first mentor and friend.
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.
In the most recent Japanese Fan Poll, Chopper is the fourth most popular character. His birthday is Christmas Eve.
Oda has stated that he uses the color pink to represent Chopper, a trait he shares with nearly all doctors from Drum Island. Oda has also stated that Chopper's favorite foods are cotton candy, chocolate, and "sweet stuff".
When creating Chopper, Oda stated that he wanted a mascot who is cute and fearless.
Powers and abilities
Chopper ate the "Human Human Fruit" (Hito Hito no Mi), a "Zoan"-type fruit which gives whoever eats it three transformations:
From Left to Right: Walk Point, Brain Point, Heavy Point
Walk Point (脚力強化 (ウォークポイント)): This is Chopper's form as a normal reindeer, useful for galloping and general travel on foot. Called "Sprint Boost" in the 4Kids English dub.
Brain Point (頭脳強化 (ブレーンポイント)): Similar to Chopper's standard form, with a small body and large head. He is smarter in this form. In addition, he can scope out an enemies weakness in this form, while he is under the influence of a Rumble Ball. In the 4Kids English dub it is named "Brain Boost" and in the video games as Smart Boost. This is also the form where he does his best strategizing, as he can utilize both human intelligence and reindeer sensitivity at the same time.
Heavy Point (重量強化 (ヘビーポイント)): Chopper's "human" form, which gives him a werehuman-like appearance and height. He uses this form to help steer the ship. Called "Heavy Boost" in the 4kids dub, and "Weight Boost" in the Funimation dub, 4kids videogames, and Viz Manga. This form imbues Chopper with super-human strength.
Rumble Ball Transformations
In addition to his normal transformations, Chopper developed the Rumble Ball, which gives its user several more transformations for three minutes by "distorting the wavelengths of transformation" Although the primary effects of the rumble ball only last three minutes, Chopper cannot consume a second ball for six hours without adverse effects, mainly the loss of control over his transformations.
Jumping Point (飛力強化 (ジャンピングポイント)): Chopper keeps his reindeer legs in a human-like form, which allows him to jump long distances. "Jumping Boost" in the 4Kids English dub.
Arm Point (腕力強化 (アームポイント)): Chopper's second most powerful form, his fore-hooves become extremely muscular in this form, powerful enough to inflict a great amount of damage. In the 4Kids English version, this is called his "Arm Boost"
Horn Point (角強化 (ホーンポイント)): Similar to Walking Point, but Chopper's forelegs get bulkier and his horns get larger and sharp. Chopper first uses Horn Point against the Mr.4 team, labeling it his "most powerful form". In the 4Kids English version it is called "Horn Boost".
Guard Point (毛皮強化 (ガードポイント)): Chopper's fur grows out and becomes an impenetrable shield against outside attacks. In the 4Kids English version it is named "Guard Boost".
If Chopper consumes three or more Rumble Balls within six hours, he grows to monstrous proportions and loses all self control. This form is extremely powerful, but also has the adverse effect of draining Chopper's life force. The only known way to reverse this transformation is by submerging Chopper in water, negating his Devil Fruit powers. Chopper changed into this form while fighting the CP9.