Jean Bart is a pirate who was originally introduced as slave of Saint Roswald's Pirate Captain Collection. He is named after real life pirate, Jean Bart. After the events caused by Luffy at the Human Auctioning House, Trafalgar Law freed and gave him the offer to serve under him. Having been freed from his Tenryuubito master, Jean Bart accepted Law's offer and joined the Heart Pirates.
During the chaos that was caused in the Auction House, Law freed Jean Bart from his collar. Having been recognized by Law, Jean Bart was offered to join the Heart Pirates. Noting that it had been a while since he was called by his name, Jean accepted to join Law. Fighting alongside Law against the Marines before them, Jean Bart implied a "thank you" to Law, for freeing him. Law counter-replied that half of his thanks belong to Mister Straw Hat (Luffy), who if it wasn't for his recklessness, he would still be a slave.

(Source: One Piece Wikia)