This Article Contains Spoilers - WARNING: This article contains major spoilers. If you do not wish to know vital information on plot / character elements in a story, you may not wish to read beyond this warning: We hold no responsibility for any negative effects these facts may have on your enjoyment of said media should you continue. That is all.
Toothless is one of the main protagonists (alongside Hiccup) of the How to Train Your Dragon franchise. He's a dragon, specifically a Night Fury, one of the rarest and most intelligent species of dragon there is. He plays a similar role to his book counterpart.
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 possesses 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. He proves himself to be a very able fighter, such as 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 seems to have 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, docile, maybe even a little teasing when not threatened. He also shows a great curiosity, imitating Hiccup's actions and mannerisms when they initially 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.
How to Train Your Dragon
Toothless is considered Hiccup's first and 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, although still being wary. 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 (corkscrews, loop-de-loops, etc), not to throw her off, but just to terrify her. After she apologizes, Toothless begins to trust her more and assumes she is now a friend, smoothing out his flight.
Toothless senses another threat the next day when he manages to hear Hiccup screaming from afar. Realizing he is in danger, Toothless manages to claw his way out of the cove (for the first time on his own) and immediately rushes 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' 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. Another example of their incredibly strong friendship is in the new short, Gift of the Night Fury, where Toothless was given the opportunity to fly on his own, without Hiccup. At the end, he trashes the new tail and throws it to some unknown area on the island and nudges the old fin that requires Hiccup to fly. This is like Toothless is saying, “I want to need you.”
How to Train Your Dragon 2
By the second movie, Hiccup and Toothless have taken to exploring the areas beyond Berk, as well as experimenting with a glide-suit that Hiccup has developed that allows him to fly alongside Toothless (an adjustment to Toothless' tailfin allowing him to maintain a glide on his own, although he still cannot steer). On one of these missions, they encounter Hiccup's long-lost mother, Valka, who reveals a series of hidden "spikes" on Toothless' back, that even the dragon didn't know about -- most likely because he lacked other Night Furies to show them to him -- which he can subsequently extend at will to increase his maneuverability in the air. These "spikes" are flat, leathery flaps, placed in "V" shape and extending from his back to the end of his tail, shortly after the saddle.
When Hiccup attempts to confront Drago Bludvist and his Alpha Bewilderbeast, he is unprepared for the Bewilderbeast's Alpha Dragon "status", which allows it to take control of Toothless on Drago's orders. Toothless is nearly made to kill Hiccup, but Stoick saves Hiccup at the cost of his own life. In pain and grief, Hiccup initially rejects Toothless when Toothless regains his own mind, allowing Drago to use his Bewilderbeast to take control of all the dragons and lead them to Berk, even riding on Toothless personally.
However, when Drago arrives back at Berk, he finds Hiccup and the other Riders already present, having managed to return home by riding the baby dragons that are outside the control of the Alpha. Still mocking Hiccup's beliefs in how to train dragons, Drago allows Hiccup a chance to "talk" to Toothless, only to be shocked when Hiccup's words and his bond with Toothless allow him to break the Alpha's control. Returned to himself and guided by Hiccup's words, Toothless not only charges a plasma bolt that saves him and Hiccup from being frozen by the Bewilderbeast's ice, but subsequently challenges the Bewilderbeast directly, breaking its hold on the other dragons and rallying them to aid him. After the Bewilderbeast is forced to retreat after Toothless breaks one of its tusks, the assembled dragons all acknowledge Toothless as their new alpha, recognizing Toothless as a worthy guardian as opposed to the fear and authority maintained by Drago's Bewilderbeast.
How to Train Your Dragon: The Hidden World
In the final film, Hiccup and Toothless meets a white female Night Fury called Light Fury who Toothless' love interest.