Kallura

Kallura is the Het ship between Keith and Princess Allura from Voltron: Legendary Defender.

Season One
When Keith discovers the Blue Lion along with fellow Garrison cadets and the formerly missing pilot Shiro, the Blue Lion activates and flies the group to Arus where they awaken Allura. She guides Keith and the other new Paladins of Voltron into locating the missing Lions and fighting against the evil Galra Empire.

During the episode Return of the Gladiator, Keith is seen smiling at Allura while he watches her interact with the local Arusians.

Later, during the battle to retake the Castle of Lions from Sendak, Allura and Keith coordinate a strategy to defeat him, with Allura accessing the Castleship's computer systems while Keith battles the Galra Commander, effectively distracting Sendak until the princess can lock him in a barrier generated by the computer - a plan the two execute near wordlessly.

After Allura uses her energy to heal an entire planet, the Paladins go on a training mission while she rests. When it turns out the mission will take longer than planned, the Paladins contact her and Lance, Keith's teammate, flirts with the princess to the point of angering her and exasperating the rest of the team with his antics. Keith, recognizing Allura's feelings and Lance's lack of focus, orders Lance to stop. Lance believes this means Keith is jealous that he is not the object of Allura's affections; this is simply Lance's delusion, as there is no indication of any romantic interest between Keith and Allura nor from Allura towards Lance, and she was insulted by Lance's objectifying and belittlement of her. Later, Keith reminds the team of Allura's weakened state, saying the struggle of their current mission pales in comparison to the princess' strained health.

As the team tries to gather information on the Galra Empire, a mission goes awry and Allura sacrifices herself to allow Shiro to avoid capture. She becomes imprisoned in the Galra Empire's headquarters. The Paladins wish to rescue her, but Keith suggests they best not make the attempt, as bringing Voltron directly to Emperor Zarkon seems like a terrible idea. He admits he is not fond of abandoning a teammate, but is thinking like a Paladin. The team attempts a rescue anyway and Keith fearlessly battles Zarkon to protect his team, give them a chance to save Allura, and possibly end the war. It turns out Keith's suggestion was entirely warranted - Allura herself is dismayed that the Paladins have brought the Lions to Zarkon, as he was the original Paladin of the Black Lion and Voltron's power must not fall into his hands.

The rescue is successful, but the team barely escapes and becomes scattered by a corrupted wormhole.

Fandom
AO3

Trivia

 * The ship is traditionally the canon "flagship" of the Voltron franchise. Legendary Defender is the first incarnation of the characters Keith and Allura that have yet to be in an established relationship or show any romantic interest in each other.
 * Previous Voltron media traditionally had Lotor obsessively in love with Allura and fighting Keith to capture her. This is also not present in Legendary Defender.