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.

In his early days, Hiluluk was a thief who was diagnosed with a fatal illness. What he believed cured him was the serene vision of a grove of cherry trees. After being cured, he came to believe that there is no disease in the world that cannot be cured. He had a complete change of heart and returned to Drum Island, the island he was born on, to help cure a dire illness.
After one year had passed, Hiluluk reluctantly kicked Chopper out of his house, as he was going to die from a terminal illness (possibly tuberculosis). Hiluluk didn't want his friend to worry about him. After Chopper learned the truth, he went on a journey for an Amiudake mushroom, in which he came back brutally beaten which deeply broke and touched Hiluluk's heart. Sadly, unknown to the reindeer, the mushroom turned out be poisonous; but Hiluluk, who did know this, took it anyway because he was so moved by Chopper's concern from him. (Hiluluk had always called the jolly roger pirate emblem a symbol of vitality, which Chopper took literally, not realizing that the crossbones symbol in the medicine book he read meant that the mushroom was poisonous.)

Afterward, Hiluluk heard news that Wapol's personal 20 doctors, the Isshi-20, were sick and rushed to the castle, only to find out it was a trap by Wapol to draw Hiluluk out and kill him. He thanked Chopper for a wonderful life just as Chopper was about to reach the castle. Hiluluk realized that his life would soon end and so, too, would his country if nothing was done about Wapol. Hiluluk truly believed that his country would be healed in time and his death would not be in vain. Thus with clear and hopeful eyes, and so as not to die from the poisonous medicine Chopper had made, he drank one of his own failed medicines and was blown up in an explosive blast.

(Source: One Piece Encyclopedia)