Appearance
Most of the time, Chopper is a toddler-sized human/reindeer hybrid, his devil fruit abilities allow him to change his appearance depending on the situation. Chopper's left antler has a metal plate wrapped around it at all times, from being broken in a fight when he was younger. The usual clothing he wears includes a pink, fuzzy top hat with a sideways medical cross (given to him by Hiluluk) and a purple pair of shorts. Chopper also has a remarkable feature that no other reindeer have, and that is a blue nose, while normal reindeers have black ones. As Oda's style evolved, Chopper has been given a more "chibi" appearance. This means that when in his Brain Point, Chopper has a larger head and eyes.
A running gag is that various characters in One Piece think Chopper is a tanuki (raccoon-dog, often simply translated as "raccoon") whilst in his Brain Point form. This is a play on the similar word "tonakai," the Japanese word for reindeer, from which the "Tony" in Chopper's name is derived. His heavy point form likewise is mistaken for a gorilla or an abominable snowman.
Lots of female characters, such as Porche, find Chopper very cute.
Oda has stated that he uses the color pink to represent Chopper, and (Oda was answering the questions as Chopper) for his smell, "how am I supposed to know?".
Personality
Chopper is naive and highly timid, often acting like a child because he doesn't know any better. In particular he is a coward at times like Usopp and Nami, his cowardice is mostly due to his childishness and overall lack of trust in his own fighting skills against the bigger foes (a contrast to Nami's self-survival based cowardice and Usopp's fear based cowardice). Chopper is more or less on a journey to define himself, seeing almost anyone on his team as a role model and in some cases, imitate their behavior, especially Usopp. He is an upright friend and companion who will try anything to overcome a task given by his team. He still has a feeling of distrust for humans from his youth and will often outright insult and claim that compliments don't make him happy to a human who compliments him while obviously being happy about it (he starts to smile and also dances a little). Chopper seems to be very forgiving of those close to him, for when Usopp wanted to rejoin the crew at the end of the CP9 arc, he called out to Luffy and Zoro, although they were ignoring him completely.
History
No Disease is Incurable
Chopper incorrectly concealing himself while spying on Nami.
When Chopper recognizes someone needs medical attention he often will start to panic and scream for a doctor, just to realize that he is one. He would rather run and hide than fight, but becomes more courageous when others are there to support him. He also has very high regards for his own profession of doctor. His dream is to become a doctor that can cure any illness, and he highly respected Dr. Hogback for the huge amount of people he had saved. When Hogback informed him of his work with zombies in order to revive the deceased, Chopper still held him in very high esteem, believing that healing the hearts of those touched by tragedy was the highest possible calling for a doctor. When he discovered that Hogback wasn't actually reviving the dead, but merely using their bodies as puppets, Chopper became furious and refused to acknowledge Hogback as a doctor anymore.
Another one of Chopper's habits is his reverse cartoon peek, where he stands behind a wall or a doorway and peeks at people, trying not to be seen by them. Unfortunately he's not hiding his body behind the wall, as a result his body is visible and only one half of his face is hidden.
A running gag is that Chopper will believe just about anything he hears, making him easy prey for Usopp's stories. He also seemed to believe Nami when she lied to Lola about being a man, even though the truth was very obvious.
Chopper was originally a reindeer with the slight oddity of a blue nose, which caused other reindeer to treat him as an outcast (always making him follow the herd from far back). However, when the young reindeer ate the Hito Hito no Mi, he was even more ostracized from his herd (and with the increase in intelligence granted by the fruit, now acutely aware of it) and struck out on his own. Unfortunately for Chopper, his attempts at communicating with the humans on Drum Island proved disastrous, he was shot by villagers who mistook him for the Abominable Snowman. Luckily, though, he was rescued by the quack Dr. Hiluluk (Hiriluk in the English version), who named him and took him in as his friend and 'assistant.' Although his deeds were of dubious medical value, Dr. Hiluluk became Chopper's role-model. Hiluluk taught Chopper his philosophy on life (that all diseases could be cured) and his strong faith in the Jolly Roger as a symbol of strength against all odds. The two went from house to house, administering their "cures" in a country where all doctors not sanctioned by King Wapol were banned.
When Hiluluk's health took a turn for the worse, Chopper found himself left out in the cold once again as the old man didn't want Chopper to watch him die. But once Chopper found out what was going on, he was determined to find something to cure his mentor, his one and only friend. He soon found what he was looking for: a special kind of mushroom with skull-and-crossbones prominently displayed in the medical book, unaware of what that sign meant medically due to Hiluluk's praising of pirates and the jolly roger. Remembering what he overheard by some villagers that a "certain" type of mushroom can heal every illness, Chopper steals Hiluluk's favorite medical book to find the miracle mushroom. Chopper's arduous journey was a success, even though he risked his life and suffered many injuries, such as a broken left horn.
Hiluluk overwhelmed with tears of gratitude, ate the mushroom to show his love and appreciation to Chopper, but Chopper would later find out from Dr. Kureha that he had given Dr. Hiluluk a fatal dose of poison. But Hiluluk was not to live much longer anyway. Declaring that he had a wonderful life (in the 4kids dub he declares he is a doctor), he committed suicide by blowing himself up outside of Drum Castle (before being killed by the poison, for Chopper's sake). As for Hiluluk's last wish, Dr. Kureha (the last remaining doctor not owned by King Wapol) took Chopper in and taught him 'real' medicine for the next 6 years. Chopper worked as hard as he could, in order to become a doctor like his idol, Hiluluk.[
