Mochi first appears in Lucky Cat Cafe after Aunt Cass returns from picking up her nephews Hiro and Tadashi Hamada. Cass, suffering from stress eating, calls to Mochi, who follows her into the kitchen.
After Hiro returns home from following the trails of his Microbots, Mochi appears, purring and rubbing against Hiro's leg when he tries to hide Baymax. After Hiro throws him up the stairs, it is quite possible that Baymax caught Mochi while calling him a "hairy baby." When Hiro arrives, Mochi dashes downstairs. When Baymax sneaks down again, Mochi and Aunt Cass are watching television. He is also shown in the end credits.
A painting of Mochi wearing a Stitch costume can be seen behind Hiro when he is talking to his aunt.
Mochi's name comes from a type of Japanese rice cake, which is usually made in a special mochitsuki ceremony, and is usually served around the Japanese New Year.
Mochi originated from the film's storyboards by story artist Kendelle Hoyer who felt that Hiro and Tadashi’s family needed a pet. Eventually, Mochi was finally moved into the film.
Mochi was originally going to have rocket boosters that were made by Hiro. Although Paul Briggs and screenwriter Robert L. Barid absolutely adored the idea, there was no way that it could be put into the movie in a way that would make sense in context with the movie. However, the idea was later recycled when Hiro and Tadashi were in San Fransokyo Tech in the background as someone's science experiment. When seeing some advertising for the movie in Japan and seeing the Japanese audience fall in love with the concept, Briggs said, "You guys do realize that Rocket Cat hasn't been in the movie for a year now?" as the idea was removed early on in production.
Although Mochi does not get rocket boosters, at San Fransokyo Tech there is a student making a pair for his own cat, which has a similar appearance to Mochi (though it is a black cat). The student and his cat can be seen when Tadashi first walks Hiro through his school's lab.
Baymax's infamous line while petting Mochi, "Hairy baby...hairy baby," has become a popular Internet meme.