In the Film
Toothless appears as a Night Fury in the film, one of the rarest and most intelligent species of dragon that there is. He plays a similar role to his book counterpart.
Physical Appearance and Personality
Toothless is much larger than his book counterpart, being much bigger in size than Hiccup. He has jet black scales covering his whole body, his sides having small light patches in a pattern similar to that of a manta ray. Toothless posseses at least two sets of wings (one main pair and two smaller ones near the base of his tail). He is missing the left of his tail fin, lost when Hiccup takes Toothless down with his Bolas-Shooting Ballista. This is later replaced by a mechanical fin that Hiccup constructs himself. Toothless' eyes are a greenish-yellow. He can narrow his pupils to slits when threatened or angry, similar to that of a cat. But when he is docile or happy, his pupils become larger and more dog-like. Toothless gets his name from his sets of retractable teeth.
Toothless proves himself to be a very able fighter. When he rushes to Hiccup's aid in his battle with the Monstrous Nightmare, Toothless quickly causes the larger dragon to back down. Toothless also shows himself superior to most Vikings, as proven against Stoick in a quick skirmish.
Toothless is very intelligent and has a dry sense of humor, similar to Hiccup's. He does not take kindly to being offended or anyone offending or endangering his human friend, but is usually playful and docile when not threatened. He also shows a great curiosity, imitating Hiccup's actions and mannerisms when they initally encounter one another. Toothless' intelligence shows many times throughout the film, being able to understand Hiccup's words and commands as well as agree with some of what he says. It is shown that he has become aware of Hiccup's crush on Astrid, even seeming to briefly tease him about it.
Relationship with Hiccup & Role in the film
Toothless is considered Hiccup's best friend in the film. Although initially enraged at the treatment he receives at the beginning of the film, Toothless' curiosity of the human gets the better of him and he soon begins to learn more about Hiccup. After Toothless lets the Viking touch him for the first time and also fit him with the mechanical tail fin, he grows very close to the boy shortly after. He enjoys spending time with Hiccup and they always fly together, seemingly in sync with one another's movements.
When Hiccup unknowingly leads Astrid to the cove, Toothless shows his protective side by assuming that she is a threat. Only through Hiccup's words does he manage to calm down enough. When Astrid threatens to tell the village, Toothless abducts her and strands her on a tree to stop her. Even though Hiccup invites her to a flight to show Toothless is not dangerous, the Night Fury is not convinced and pulls off daring acrobatic moves, not to throw her off, but just to terrify her. After she apologizes, Toothless begins to trust her and assumes she is now a friend.
Toothless senses another threat the next day when he hears Hiccup screaming from afar. Realizing he is in danger, Toothless manages to claw his way out of the cove and immediately rush to Hiccup's side to protect him from the dragon he is supposed to kill. He refuses to leave his human's side when the Vikings eventually get in on the fight and gang up on him, managing to fight them off and have Stoick at the mercy of his flames. Only when Hiccup screams at him to stop does he let his guard down long enough to be captured.
Toothless is chained up and used a compass to reach the Dragon's Nest. However, as he observes the other dragons flying away, he begins desperately trying to free himself as the Red Death suddenly bursts out of the mountain of the Nest. The ship he is chained to is set alight, and both he and Hiccup are thrown into the water when the ship breaks up. Stoick, however, saves both of them and Toothless takes on the larger dragon with Hiccup by his side.]Toothless' tail at the end
Toothless' bond with Hiccup is shown to be very strong indeed when his fake tail fin is set on fire in the ensuing battle. When Hiccup is knocked off his back, Toothless bravely dives into the exploding body of the Red Death to save his human friend, wrapping Hiccup in his wings to protect him as they crash. Toothless is overjoyed when Hiccup wakes from the battle later, licking him in the face playfully, like a pet would, knocking all sorts of things over while jumping around Hiccup's home happily, and helps him to walk with his new prosthetic leg as a sign of their friendship. Toothless gains a new tail fin at the end of the film, given to him by Gobber, and takes off into the skies with Hiccup once again.
Role in the series
Toothless is a dragon ally to Spyro and Spongebob in their efforts to defeat the leage when needed.