Chopper is a reindeer that ate the Hito Hito no Mi which is a devil fruit that allows its user to transform into a human hybrid and a full human at will. He came from Drum Island where he learned how to be a doctor, which also makes him the only member of the Straw Hat Pirates who was born on the Grand Line.
He is the sixth member of the crew and the fifth to join Luffy, as well as being the youngest member on board. He has a bounty of 100 due to being mistaken for the crew's pet.
Most of the time, Chopper is a toddler-sized human/reindeer hybrid, but his Devil Fruit abilities allow him to change his appearance depending on the situation. Chopper's left antler is braced at the base by a metal plate because it was reattached after being broken during his search for the Amiudake when he was younger (where he thought the Amiudake can cure Hiluluk's disease) and ran into the leader of his old herd, who severely injured him. He usually wears a large pale red/pink fuzzy top hat with a sideways medical cross (given to him by Hiluluk) and a maroon pair of shorts. He also sometimes wears a blue backpack that has the same sideways medical cross as his hat. He also has a remarkable blue nose. As Oda's style evolved and many readers commented on how cute Chopper was, Chopper has been given a much more "chibi" like appearance, meaning that when in his preferred transformation (which he calls Brain Point), he has a larger head and eyes and a less defined muzzle than earlier in the series. Many female characters, such as Nami, Robin, Makino, Porche, Shakky, and Vivi find Chopper very cute. There are even males that find Chopper cute such as Foxy and Breed.
A running gag is that various characters in One Piece think Chopper is a tanuki (raccoon-dog, often simply translated as "a raccoon") while in his Brain Point or hybrid form. The word "tonakai", which is the Japanese word for "reindeer", is where the "Tony" in Chopper's name is derived from. His Heavy Point or human form is likewise mistaken for a gorilla (or an abominable snowman on his home island).
After two years, Chopper is significantly leaner. He is less potbellied than he was in earlier seasons and his arms/forelimbs are also skinnier. It is arguable that since leaving the cold climate on Drum Island, he has lost significant body fat. The color of his fur has changed from brown to gold, and according to Nami (when she and Chopper reunite on Sabaody), the texture has gotten softer.
Chopper now wears a blue cap that has a ring of dots around the circle, while the cap has buckles hanging from it. He wears this over his old pink hat. He also wears a white and yellow vertical-striped tank top, which he takes off when he transforms into Heavy Point, and an orange pair of shorts. Aside from being slightly slimmer and having his fur changing from dark brown into a light brown color, Chopper's Brain Point form has not changed. His Walk Point (reindeer) form has grown considerably, now being roughly as tall as Usopp. His horns are larger, he has more muscle, and his fur is longer. His Heavy Point has become exceedingly larger and more muscular, being almost the size of Dosun's. He also developed new transformations through his research during the timeskip, which he debuts on Fishman Island, including Kung Fu Point (which he uses to defeat Neptune's soldiers) and a revised Horn Point (which he uses against Daruma).
While on Punk Hazard, he was given a black coat by Kin'emon using his unnamed Devil Fruit ability. He was later dressed up in samurai attire made from the same ability when the crew had departed from Punk Hazard.
At the start of the Dressrosa Arc, Chopper wore camouflage clothing. After the crew split up, he changed his headgear for the first time since the timeskip, putting on a football helmet with holes for his antlers that has three horizontal stripes running vertically down the middle, with two small ones beside one larger one, as well as large stars on each side of the helmet. He also put on a zippered coat with the same stripe pattern running horizontally all around the middle of the coat, with inverse coloration of the stripes on the helmet and a pattern of stars inside the large stripe on the coat.
During his stay on Zou, Chopper's outfit consists of his original timeskip hat with a crown on top. He wears no shirt and has a long open coat with fur and fancy jewelry attached. Upon leaving Zou, Chopper wore an open doctor's coat.
During the Whole Cake Island Arc, he wore a yellow overcoat and had a pink scarf wrapped around his hat. Afterwards, when meeting with the Fire Tank Pirates he wore an outfit similar to Luffy's dark suit jacket with an ascot and dark shorts.