Hatchan (はっちゃん, Hacchan, called "Hachi" (ハチ) by most characters) is an octopus fish-man. Hatchan is not very bright but a skilled swordsman, using rokutōryū ("six sword style"). He fights with Zoro during the Arlong arc. Despite Zoro suffering a fever and severe blood loss from his unhealed wounds (which he got eariler fighting Mihawk), Hatchan is still defeated by him. Outside of battle, he is extremely helpful and friendly (though it should be noted that he can be just as cruel to humans as the other crew members are), addressing Zoro politely when they first met, and giving him a ride of a bay on his back. Later in the manga (in a title page arc that was never animated), Hatchan goes on many adventures, meeting a mermaid, that ends with him opening a floating takoyaki restaurant.

In Hatchan's Sea-Floor Stroll, it's revealed that Hatchan, along with Chu and Kuroobi, planned to open a takoyaki store when they were younger. His nickname Hachi could also mean eight in Japanese, a reference to him being an octopus fish-man.

Hatchan's name is quite strange. In the Japanese language, chan is a diminutive suffix, added to the end of someone's name, giving it the meaning of "cute" or "little". However, Hatchan insists that his name is Hatchan and that Hachi is just a nickname. When questioned on this, Oda replied that the chan is the same as the last name of "Chan" common in Chinese, rather than the suffix -chan.

(Source: Wikipedia)