Opalvira

Opalvira is a femslash ship between Kuvira and Opal Beifong from The Legend of Korra ship

Canon
The events in Book 4 are important to this relationship though their relationship begins earlier on. Opal and Kuvira are connected through their relationship to Suyin Beifong, though the extent of their relationship is ambiguous.

Kuvira was the guard captain in Zaofu and a dancer on Opal's mother dance troupe. The first time the audience was able to see Kuvira and Opal interact, however, was in Book 4 where it is clear that Opal and Kuvira's relationship is full of animosity. Opal sides with her mother regarding Kuvira abandoning her responsibilities in Zaofu and taking Baatar Jr. to 'unite' the Earth Empire. Meanwhile, Kuvira is focused on swaying Opal to her side while hiding the 'hard truths' of her regime.

The second time they interact is when Kuvira moves to invade Zaofu during Kuvira and Korra's first fight. Opal's bitterness towards and past knowledge of Kuvira also reveals itself when she advises Korra that Kuvira is trying to frustrate her into making a mistake. Kuvira wins the battle and takes Zaofu along with Opal's family, while Opal flies away. The moment is reminiscent of that of Opal in Book 3 flying away to begin her training as an air nomad while Kuvira, in one of the few moments she's present in Book 3, calls for the domes to be secured.

The third time Kuvira and Opal arguably could have interacted was when Kuvira attempted to kill Zhu Li, and by extension Opal and Bolin, a death only prevented by Baatar Jr. However, Opal and Kuvira do share a number of insights about each other and do generally behave in ways which suggest a deeper relationship. They are uniquely able to pinpoint each other's insecurities, for example.

During the battle for Zaofu, Kuvira suggests that Bolin and Opal are growing apart, something Opal worries about in the first episode of Book 4. Opal, on the other hand, is able to name Kuvira's exact fatal flaw which Korra is able to empathize with at the end of Book 4 - namely that Kuvira's motivation is not equality or a stronger Earth Nation but control. Furthermore, despite Opal calling for Korra to put Kuvira out of commission repeatedly, when Korra is likely about to kill or gravely injure her Opal steps forward, with an expression that suggests that she cares for Kuvira more than she lets on. One of the few times Kuvira smiles in the entirety of the series is when she is interacting with Opal in the beginning of Book 4 while she's attempting to cajole her to her side.

Fanon
Ultimately, most people began shipping these two based on the first episode of Book 4 when Kuvira put her hand on Opal's shoulder since this is the only moment where they actually interact. However, there are many reasons to enjoy the dynamic in cannon and in more fleshed out fannon. Some people believe that Kuvira lived in the Beifong household, and had a pseudo-sisterly relationship with Opal and the other Beifong children including Baatar. Others believe that Kuvira merely lived in the city itself and had more of an acquaintanceship with the Beifongs with the exception of Suyin. Others believe a combination of the above, while others choose to focus more on the possibility of their personalities in a modern AU setting devoid of the politics. However, there's much to be enjoyed about in the dynamic of their relationship, as ill-defined as it is in cannon, with the clashing of fierce and opposing moralities.

Trivia

 * The name for this ship was coined by fanfiction user, IamthecreatorofOpalvira.