Jack is the Icon of Halloween on Gaia. With his pumpkin head and tattered clothing, he prowls around handing out items to Gaian users in exchange for candy gained from the shopkeepers. Only appearing once a year to celebrate his holiday, he's a rare and welcome sight. He's had an avatar change every year since he first appeared, and in 2005 he first appeared in Gaia Towns, revealing his true stature and height. He's apparently about three or four times as tall as a normal avatar, with cloven feet and a menacing gait.
After the fiasco of Halloween 2005, we learned a bit more about Jack, courtesy of Santa Claus. Apparently, he's the cause of the Easter Bunny's cybernetic parts, as he once injured the bunny so much that his face was permanently deformed, necessitating the cyborg parts we see now. He did this while saving Santa from the gang of holiday characters that was beating him up. Jack also showed us his ruthless side, punching Santa clear through the linklist and presumably killing him. What caused this attitude change towards Santa, we don't know.
The very next plot update gave us yet another change in Jack's personality when he softened and went back to being Santa's best friend after an epic duel. When the G Team failed to halt the duel, a second effort was made by the newly introduced Masque, who flipped the Icons apart with ease.