Kai was born in a kingdom called Snoob, where he grew up alongside his brother, Karl. At some undetermined time, Kai left Snoob and came into the service of King Agnarr of Arendelle. He nevertheless remained in touch with his family in the years that followed, until Agnarr ordered for the castle gates to be closed, thus terminating any contact between the outside world and the castle's inhabitants.
As the chief assistant of Agnarr and his wife, Queen Iduna, Kai was diligent and loyal. When not tending to the king and queen, Kai would act as the caretaker of their daughters, Elsa and Anna. Following Agnarr and Iduna's deaths, Kai would continue to serve Arendelle as Queen Elsa’s attendant, and later Queen Anna’s.
Kai is featured very briefly in the film. At the beginning, he is one of the few servants who remains within the castle by orders of King Agnarr and Queen Iduna when the castle is locked away from the rest of the kingdom. During "Do You Want to Build a Snowman?", Kai can be seen aiding the couple prior to their ill-fated sea voyage. When the king and queen perish in a storm, Kai and another servant are seen veiling the royal portrait.
When the castle gates finally open for Elsa's coronation, Kai introduces The Duke of Weselton to the royal siblings at the coronation's ball, but accidentally mispronounces his home's name as "Weaseltown", leading him to be harshly corrected by the Duke.
After Elsa finally returns to Arendelle and lifts the winter curse, Kai is made aware of Hans' fate by the French dignitary. Kai then informs the Duke that Arendelle will be cutting ties with Weselton permanently, made official by Elsa herself. Much to the Duke's dismay and irritation, Kai pronounces his kingdom as "Weaseltown" once again, this time intentionally.
Kai is last seen in the film at the castle courtyard, cheering Elsa on as she creates an ice skating rink for her subjects' enjoyment. Afterward, Kai can be seen skating along the ice with a servant woman.
In the featurette, Kai is first seen tending to Christmas preparations in the castle, when he receives a surprise visit from Olaf. He is later seen during "Ring in the Season", as Anna and Elsa prepare to bring in the Yule bell to kick off the holidays. Near the climax, Kai, Anna, and Elsa learn from Sven that Olaf has gotten lost in the woods and is being chased by hungry wolves. When they find the snowman, Olaf reveals that he had been on a mission to find holiday traditions for Elsa and Anna. This kicks off a party in the woods, attended by all of Arendelle.
- Kai is named after one of the protagonists of the original Snow Queen tale by Hans Christian Andersen, by which Frozen was inspired. In the original story, Kai (or Kay) was the imperiled best friend (brother in some interpretations) of Gerda, a brave heroine determined to rescue him from the Snow Queen.
- Kai's voice actor, Stephen J. Anderson, is a longtime director, story artist, and animator of Walt Disney Animation Studios.