Luffy and the rest found Chopper while looking for a doctor to help Nami in Drum Island. They soon found out Chopper is a very very special being. Being born a blue-nose reindeer, he was isolated from the rest of his species. One day, he ate a devil's fruit (the human human fruit) that increased his intelligence to that of a human being and allowed him to transform into some kind of human-reindeer hybrid at will. However, now not only reindeers rejected him, but he was also called "monster" by the humans, and so he remained alone.

Till one day a man, Dr. Hirukuk, took Chopper under his care, who out of admiration for the man decided to follow his steps and become a doctor. Unfortunately, Hiruluk had an incurable disease which ultimately took his life. Chopper then was taken by Dr. Kureha, an acquaintance of Hiruluk.

Resigned to never finding his place in the world, Chopper finally found a home with Luffy and his crew. After freeing Drum Island from the evil King Wapol, Chopper left and became the Straw-hats doctor.