- Cass is the overworked but always-supportive guardian of brothers Hiro and Tadashi. The owner of a popular San Fransokyo bakery and coffee shop, Aunt Cass is proud, ebullient and thinks the world of her two genius nephews. She's always good for a laugh and ready with a hug, tirelessly there for support and a great home-cooked meal.
Cass is a caring woman who loves her nephews above all else. She is very loving toward them, but knows when to be strict and firm when necessary, and can get very frustrated and stressed because of them. This is shown when she picks them up after they are released from prison. She hugs them and asks if they are okay before lecturing them semi-strictly about how much they worried her.
Cass seems very excitable, talkative and is usually in a happy mood. She is also understanding and did what she could to help Hiro through his depression after Tadashi's death.
She supports and encourages the decisions of Tadashi and Hiro while wanting what is best for them, as she encourages Hiro to go to college as it is what Tadashi would have wanted. She is proud of her nephews and is not afraid to show it.
Aunt Cass does not have a very big role in the film. However, she is clearly the mother figure for both Hiro and Tadashi. She first appears when she goes to the police station to pick up her nephews after they are arrested and released on charges of involvement in illegal bot-fighting. She hugs them and asks if they are okay before proceeding to lecture them about how worried she was for them and their irresponsibility. She states how she took them in despite not knowing much about children, not being the best mother figure and possibly needing to pick up a book on parental guidance. She then proceeds to go to the kitchen to pick up some snacks to calm her stress and goes upstairs with the family's cat Mochi. Tadashi tries to talk Hiro out of bot-fighting before Aunt Cass eats everything in the cafe due to her stress.
She is next seen at the San Fransokyo Institute of Technology convention supporting Hiro and stating how proud she is of him and Tadashi to the audience after his display. In celebration of Hiro getting admitted into the Institute, she offers to cook dinner at the cafe for Hiro, Tadashi and the rest of their friends free of charge. Tadashi asks her if he and Hiro can have some time by themselves, telling her they'll catch up, and she accepts. However, shortly after she goes to the car with everyone else, Tadashi dies in a fire at the convention hall.
She is shown at her house mourning Tadashi's death, being comforted by Tadashi's friends Fred, Honey Lemon, Go Go Tomago and Wasabi and also attends his funeral with Hiro and the others. She brings Hiro his food as he is too upset to leave his room and continues to mourn Tadashi's death, for about a month or so. She notices his plate is untouched and tries to console Hiro and convince him to register for classes at San Fransokyo Tech as that it is what Tadashi would have wanted, but Hiro refuses to budge and she decides to give him space. Later, when Hiro notices Baymax leaving his room to track where the microbot is leading, Hiro follows but is caught by his Aunt Cass. He lies to her by saying that he is going to register for classes and she hugs him, happy to see that Hiro is finally out of his room and going to apply for college. She gives him a hug and a quick kiss and then another hug before letting him go.
When Hiro returns home after nearly being killed by Callaghan and trying unsuccessfully to report the incident to the police, Cass notices that he is home and is preparing wings for a special dinner in celebration of Hiro's "first day at college" and wants to hear about it. However, Hiro dodges this by saying that he has a lot of homework and assignments to catch up on from college and takes the wings to his room to hide Baymax from her. Hiro then sneaks past her while she is watching Frankenstein to go to the garage to create armor for Baymax and download a fighting chip so Baymax can fight back against Callaghan. (Her reaction to Hiro's absence and her suspicions of his whereabouts are not shown).
She is not seen again until the end of the movie when Hiro leaves for San Fransokyo Tech with his friends and they hug, and Hiro asks for one last hug as he takes the lunch she prepared and leaves for the college with her smiling as she watches him leave.
In the end credits, she is shown being acquainted with Baymax, implying that Hiro introduced her to him once Baymax would no longer expose Hiro's scheme of avenging his brother. However, it is unknown if she knows about the Big Hero 6, though it would seem unlikely. She is also seen in a newspaper photo attending a ceremony of a building dedication in Tadashi's name alongside Hiro, Mochi, Baymax and the other members of the Big Hero 6 team.
Cass appears in the follow-up series as a recurring character, still oblivious to Hiro's double life as a superhero. Though usually playing a minor role, she has sometimes become an unwilling participant in the schemes of various villains. For example, in "Food Fight", Cass accidentally joins an underground cooking competition; despite her eventual love for the sport, her winning streak gets her into trouble with a rival chef/supervillain named Momakase. In "Aunt Cass Goes Out", Cass meets Alistair Krei while visiting Hiro at school, and develops a relationship with him. Around that time, unfortunately, one of Krei's affiliates hatched a scheme against Krei, in which Cass was unintentionally dragged into. In both episodes, Cass has been implied to have some interest in having a life outside of running the café and parenting Hiro—ironically, the events of both episodes left her sour to the idea.
In Ralph Breaks the Internet, Aunt Cass appears on an advertisement for "sassy housewives that want to meet".
- According to the 2013 Rotoscopers video and early development stages of the plot, Cass was originally Hiro and Tadashi's mother instead of their aunt. The reason for this change is yet to be verified, although it remains in the Japanese text of the manga adaptation, Baymax along with some elements left over from the early stages.
- She has a stress eating and cooking problem.
- In early concept art, as well as the initial model renderings, Cass had a noticeable tattoo on her left arm. In the final film, the tattoo is completely gone. The reason for this has not been confirmed.
- She has stated that she has not always been the best of role models despite seeming to be an encouraging and loving guardian to her nephews.
- She originally had a larger role in Hiro's anxiety complex over Tadashi's death, and the film's concept art shows that she was present with Hiro in the "Tadashi is here" scene.
- It is revealed in "Aunt Cass Goes Out" that she has never dated before Alistair Krei and, by extension, has never been married.