Toothless and Light Fury's Relationship

"Can you believe it? Toothless has a girlfriend!"

- Astrid Hofferson

Toothless and Light Fury's Relationship is the strongest romatic relationship between dragons seen in the franchise.

How to Train Your Dragon: The Hidden World
During a raid to free captured dragons from the Warlords, Toothless had sensed the presence of a dragon in the cage he walked next to, but when he looked to see the dragon there wasn't one to be seen and was then called by Hiccup to retreat with the dragons they have managed to save. While the camouflage ability of the Light Fury prevent them from knowing that they had left one behind. That mistake led to the hired dragon hunter, Grimmel the Grisly, to use the her as the unwilling bait for Toothless, since his species are known to mate for life; and Light Furies are a sub-species of Night Furies.

When Toothless properly meets the Light Fury on Berk, she warns him of the trap that was set for him, as they circle around it while looking into each other's eyes. While the two furies were quickly forming a strong connection with each other, the presence of Toothless' human friends frightened the none human trusting dragon away and because Toothless can't fly on his own, he couldn't fly after her. The next, Toothless showed signs of being completely smitten with the Light Fury, from watching other dragons doing their mating dances and his own shadow reminding the Night Fury of the Light Fury. The white scaled fury was also fund of Toothless, which is why he followed him when he and the rest of Berk went in search for the Hidden World, but their different views on certain kinds of humans prevented them from being together for a long period of time.

After finding the island that many have began to name New Berk, the Light Fury approaches Toothless at night fall and leads him away from the humans so they could be safe and alone to properly preform their mate dances. Toothless wanted to be her mate as well, but he didn't know what his kind's dance is and had to rely on the dance he saw and the moves Hiccup subjects from the shadows. Some of Toothless' dance moves didn't sway the Light Fury and was beginning to lose interest in Toothless, until he drew a drawing of her, and even though he didn't like her stepping on it, the moment they looked into each other's eyes again the angry disappeared, but after the Light Fury sees Hiccup she flies away while a flightless Toothless could watch her disappear, again. Realizing that they couldn't properly spend time together in the air without frightening her away with his presence, Hiccup rebuilds the self-flight tailfin so Toothless can have that romantic flight with her the right way.

The two dragons are reunited as they soar into a storm together. The Light Fury shows Toothless how she is able to cloak her entire body by flying into a plasma explosion and asks him to do the same. After a couple of failed attempts, Toothless unknowingly channels lightning into his body to amplify his plasma blast, which allows him to become invisible. As the two dragons ascend above the storm, Toothless licks the Light Fury on her head, which prompts her to swoon over him. They then fly together towards the Hidden World.

As the dragons of the Hidden World accept Toothless as their Alpha, Toothless and the Light Fury dance together to officially become mates. The tender moment is inadvertently interrupted when Toothless realizes that Hiccup and Astrid are being chased by the other dragons, forcing him to leave his mate behind.

Toothless is initially saddened that he has to leave the Light Fury behind, but becomes ecstatic when he sees that she has followed them back to New Berk. The reunion is cut short when Grimmel captures the two of them and forces all of Berk's dragons to surrender themselves to the Warlords.

During the battle, Grimmel uses Deathgripper venom to enslave the Light Fury, Toothless and Hiccup chases after them but they are unwilling to beat the hunter while he is controlling her. Toothless then uses his cloaking technique to electrocute Grimmel's Deathgrippers that are surrounding him and Hiccup, causing them to fall to their deaths, and fly around Grimmel as Hiccup glides off to rescue the Light Fury. Grimmel manages to tranquilize Toothless midair and Hiccup, realizing he cannot save Toothless by himself, frees the Light Fury instead and tells her to save Toothless before falling down with Grimmel. The Light Fury successfully rescues Toothless before coming back to save Hiccup,while allowing Grimmel to fall to his death.

The battle won, back on the island, Hiccup fully realizes that the dragons will likely never be safe in the human world and that the Hidden World is where they truly belong. With great sadness, Hiccup and Toothless bid farewell to each other as all of the Berkians set their dragons free. Toothless and the Light Fury then lead the dragons to the Hidden World.

About a decade after the dragons left New Berk, Toothless and his mate have three offspring, each inherit the mixed characteristics of their parents, and visited Hiccup, Astrid and their two children, Zephyr Haddock and Nuffink Haddock, sail across the sea to visit Toothless, the Light Fury, their three offspring and Stormfly at the edge of the Hidden World; Hiccup vows that until mankind can coexist peacefully with dragons, the dragons will stay hidden while the Berkians guard their secret.

School of Dragons
 In The Hidden World Expansion, the player has to lead Toothless to the Light Fury. Light Fury and Toothless Then fly down into a cavern.

Quotes
"You belong there, with her."

- Hiccup

"He's all yours."

- Hiccup

"I was so busy fighting for a world that I wanted... I didn't think about what you needed."

- Hiccup

Trivia

 * According to Richard Hamilton, the Light Fury is around the same age as Toothless.
 * The fact that the Light Fury and the Night Fury are closely related implies that dragons are members of 'families', like many animals in real life (I.E., crocodiles, alligators, and caimans are all related through the crocodilian family).
 * Both species, in fact, seem to be in the same genus, like lions and tigers, as they are able to produce hybrid offspring.
 * Coincidentally, the two species also share the exact same stats.