In the book, Fishlegs is small, scrawny, and even less valiant than Hiccup. In the book, Fishlegs was unable to capture a dragon as part of his test, so Hiccup gave the dragon he had captured to him to pass. Hiccup later manages to capture a Garden Dragon which he later names Toothless where as Fishlegs keep the Basic Brown named Horrorcow that Hiccup has originally caught.
In the film adaptation, Fishlegs is quite the opposite to his book counterpart. He is very big and chubby and often spouts off information about dragons whenever he can, usually as if he were describing the stats of a creature in an RPG. This annoys the other teenagers frequently during their sessions at Dragon Training.
Later, Fishlegs learns how to fly on a dragon from Hiccup and ends up flying and owning a Gronckle. His analysis of dragons becomes invaluable for the rest of the Vikings when he identifies the characteristics and weaknesses of the Red Death.