As a baby, Hiccup was stolen away in place of his mother and brought to the queen of a nearby nest. With her magic she changes his physical form into that of a dragon for him to have a "pure life"; away from humanity. Years later while visiting Berk with his guardian, a Nightscale he calls Toothless, they are attacked and separated leaving Toothless captured and Hiccup alone...

Hiccup couldn't deny fate; soon the Vikings of Berk would discover Toothless- his best and only friend. With careful preparation, he fakes his own death and leaves Berk, with no intention of ever returning. But even as he grows into a powerful man and meets the woman of his dreams, Hiccup knows he cannot simply forget the place of his birth. After all, he's their only hope.. (HxOC)

It is in the nature of Guardians to forget. Those brought back by the Light must make a conscious effort to remember, and in the end, chasing down your memories can be an unwise and dangerous thing to do. Still, if you're determined to go haring off into the perilous and murky depths of your past, tampering in things best left forgotten, you might as well bring some company.