As Cassandra and Elsa are from different series, they have never met in canon. Even though Rapunzel and Eugenes' cameos in the first Frozen film hints that their magic filled Disney worlds might be connected or linked to one another.
Both Cassandra and Elsa are young women who had other people plan their lives for them since they were little, and are close with another young woman who are filled with joy and a bit more blessed than the two feel in their ends. Despite Elsa being the heir of Arendelle and Cassandra being the loyal daughter of Corona's Caption of the Royal Guards, Elsa's uncontrolled ice magic and Cass's handmaiden duties makes them feel that the world doesn't wish for them to be happy. Until Elsa manged to regain control of her powers, and Cassandra's friendship with Rapunzel has allowed Cass to make her duties to the princess a bit more fun than they were before Rapunzel retuned to the kingdom. When their long held feels of fear and envy got the better of them and had gotten other people to view them as villain-like figures, apart from the ones who refuse to give up on Elsa and Cassandra, it led to them fleeing from a type of kingdom and the people they could no longer face.
Because Elsa lost control of her powers and was beginning to freeze anything she touched, her father gave Elsa a pair of gloves to help her conceal and not to the magic until she is able to regain on control of it again, while Cassandra had suffered a great injured to the hand she used to touch a life draining, entranced Rapunzel and because of that she began to wear a metal gauntlet to both cover up and protect it. That gauntlet was later replaced with a dark blueish glove/gauntlet that was placed upon her injury, when Cassandra's body was magically altered by merging herself with the Moonstone opal and gained its powers, while her new magically new clothes are made out of the same midtrial as the black rocks. Elsa also has two sets of clothes that were made and placed upon her through ice magic, and her first dress was light blue and is of a similar shade to Cass's new blue hair color. Like Elsa's ice powers react to her emotions, Cassandra is able to use the Moonstone's power and control over the black rocks through negative emotions like anger and fear, that allowed her to create the red rocks of fear. When Elsa had un-antenatally revealed her powers to Arendelle, her fear had caused her magic made ice to look fearsome to others. In the same way that the color and sharpness of the Moonstone's rocks causes people to feel nervous around them. The two have also used their magic powers to create both a bridge and a castle/tower to serve as their new home. As Elsa sang "Show Yourself" in Frozen II, she leaps from the ice pillars she made with her magic, Cassandra has a similar leaping from one pillar to another scene with the black rocks while singing her end of "Nothing Left to Lose" during the events of "Cassandra's Revenge".
When Elsa and Cassandra went on a journey with their friends, where one of them is a type of animal that can be ridden on, and came across a place that shows visions of the past, they learn that they are related to villains who had pretended to be kind and caring people to the people they later hurt, King Runeard and Mother Gothel. Elsa had also lost her mother and before her death, Elsa saw her mother leave the place they both called home. Cassandra uncovering who her mother is had caused her to go down a dark path, while Elsa chose to undo the damage her grandfather had gone of the Enchanted Forest. Cassandra's journey with Rapunzel had also gotten her to come a forest that is filled with many creatures, just as the Enchanted Forest is home to four elemental spirits. The two also have their own childhood moment and placing a pillow over their ears, while trying to sleep, scenes.
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Due to the popularity of the GiveElsaAGirlfriend movement and both Frozen II and the second and third seasons of Tangled: The Series revealing Elsa and Cassandra to have a few similar traits to each other, that could have the two women get along with each other as either lovers or friends. Along with fans beginning to compare the two with one another, due to the bonds they share with Rapunzel and Anna, how Cassandra and Elsa felt when they sang their duet songs, "Crossing the Line" and the reprise of "For the First Time in Forever", and Cassandra gaining herself magical powers while wearing blue colored clothing. Black and Blue were also Elsa's earlier main colors in one of the original layouts of the first Frozen film, from when she was first set to appear as the antagonist.
On AO3, the ship has 50 fanfics, while fanfiction.net has 8 with only 4 placed under pairing. Along with it having a small fanbase on Tumblr.