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, and he was shot by villagers who mistook him for The Abominable Snowman. Luckily, though, he was rescued by the quack Dr. Hiriluk (Hiruluk in the Japanese version), who named him and took him in as his friend and 'assistant.' Although his deeds were of dubious medical value, Dr. Hiriluk became Chopper's role-model. Hiriluk 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 Hiriluk's health took a turn for the worse, however, 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 that he would find something to cure his mentor and only friend. He soon found what he was looking for: a special kind of mushroom with a skull-and-crossbones prominently displayed in the medical book, unaware of what that sign really meant due to Hiriluk's praising of pirates. Remembering what he overheard by some villagers that certain special type of mushroom can heal every illness, Chopper steals Hiriluk's favorite medical book to find the miracle mushroom. Chopper's arduous journey was a success, although he risked his life and suffered many injuries, such as a broken left antler.

Hiriluk 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. Hiriluk a fatal dose of poison. But Hiriluk was not to live much longer, anyway. Declaring that he had had a wonderful life, he committed suicide by blowing himself up outside of Drum Castle (before being killed by the poison, for Chopper's sake), where he wanted to heal the sick before he died. As per Hiriluk'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, Hiruluk.

Arrival of the Straw Hats

However, Chopper's isolated life in Drum Castle (which he and Dr. Kureha annexed after the King Wapol fled) would one day come to an end with the arrival of Monkey D. Luffy and his pirate crew. Chopper, naturally distrustful of humans, was not pleased, especially given Luffy and Sanji's desire to cook and eat him. But when Chopper showed both his medical powers and his skills in battle against the returned forces of King Wapol, Luffy offered (forced) him the (as-yet unfilled) position of Ship's Doctor in his crew. While Chopper was at first unwilling to leave, he remembered Dr. Hiruluk wishing Chopper to sail around and seeing the world on his own and with his desire to broaden his medical horizons, he decides to join Luffy and his crew. Fleeing the castle with the Straw Hat Pirates, Chopper was given a send-off of gigantic proportions: Kureha had turned the snow into Sakura blossoms, and the cliff where the castle stood became the sakura tree the very same that Hiruluk had researched over 30 years that would one day, cure the people of Drum of their "sickness of the heart", passed to Dr. Kureha as one of his last request before death. Overcome with tears, Chopper made a fond farewell to his birthplace and mentors, setting out to sea for the first time in his life.