Hiluluk's main dream was to cure the people on the island of their frozen hearts which had formed because of Wapol's selfishness. After Wapol declared any doctor not under his rule to be outlaws, he and Doctor Kureha were the only free doctors left. However, unlike Kureha, Hiluluk ended up doing more damage than good to his patients by accident.

He found Chopper one day while traveling home in a blizzard. At first Chopper tried to knock him away, but Hiluluk earned his trust by showing that he didn't have any weapons or intention of hurting Chopper. From then on, the two slowly formed a sort of father/son relationship, traveling (and in a way terrorizing) the island trying to cure patients and generally resulting in them having to literally run for their lives from angry patients or their associates.

(Source: One Piece Encyclopedia)