"A very, very long time ago, a boy in a small town saved his sister and became Jack Frost. The man in the moon knew that someone as brave as this boy should become a Guardian… when the time was right. For a long time all Jack knew was his name. Sure, he loved creating fun for children in the form of snow days, snowball fights, and general pranks of all kind, but he also wanted to know who he was and where he came from. The answers followed when he was finally called to become a Guardian and help defeat Pitch. At first he was reluctant to join the ranks of the Guardians. After all, Jack liked to have fun, not work hard. But it wasn’t long before this master of mayhem discovered his true nature- a hero. In other words, Jack Frost is the perfect Guardian." 
From the book “Guide to the Guardians”
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"A very, very long time ago, a boy in a small town saved his sister and became Jack Frost. The man in the moon knew that someone as brave as this boy should become a Guardian… when the time was right. For a long time all Jack knew was his name. Sure, he loved creating fun for children in the form of snow days, snowball fights, and general pranks of all kind, but he also wanted to know who he was and where he came from. The answers followed when he was finally called to become a Guardian and help defeat Pitch. At first he was reluctant to join the ranks of the Guardians. After all, Jack liked to have fun, not work hard. But it wasn’t long before this master of mayhem discovered his true nature- a hero. In other words, Jack Frost is the perfect Guardian." 

From the book “Guide to the Guardians”