Jackson Overland "Jack" Frost is the main protagonist in Rise of the Guardians and an immortal supernatural being much like the Guardians. Unlike the others, however, he is a loner, the classic rebel without a cause, sarcastic and mischievous. As the manifestation of winter, Jack Frost is capable of manipulating ice and snow. He is the spirit of mischief and chaos personified, but when he discovers the purpose behind his powers, he will become a true Guardian, representing "Fun".

Jack Frost is a mischievous teenage hellion who has no interest in being bound by rules or obligations, and rather instead favors spreading his winter magic for the sake of the personal amusement of himself and children. Despite these traits, Jack is also gentle and generous.

The source of Jack's anti-social behavior towards the Guardians and refusal to adhere to rules was mostly because of his frustration with the Man in the Moon never answering his questions about who he was or why he was created, and also because no human, child or adult, was able to see him for centuries for no one had believed in him. But as Jack spends time in the Guardians' company, each individual would have an effect on him that would change him from a mischief-making winter spirit to a full-fledged Guardian in his own right, as well a hero.

It was his refusal to give up in the face of overwhelming odds that enabled him to help one child, Jamie Bennett, keeps his beliefs in all of the Guardians as well as finally being able to see and believe in Jack Frost. Jack's desire to protect Jamie, as he once did his sister, helped him realize his center; Fun.

Jack proudly accepted the duties and responsibilities as the Guardian of Fun. His purpose, is to spread the feelings of fun, joy, and happiness to bolster the faith children have in the Guardians.

Even before he officially became a Guardian, Jack displayed some pride in what he does, particularly the Snow Days he makes for children to have fun.