Mushi Sanban (三番虫) is a 5-year-old Kindergartner, who is the youngest sister of Kuki Sanban, the daughter of Kani and Genki Sanban, and the love interest of King Sandy. Like her sister, Mushi is very childish and overly happy, but beneath her outward personality has a very sinister and conniving personality, despite her age. She also looks like the younger version of her sister, albeit with a different hairstyle.
In Operation: K.A.S.T.L.E., she and her sister are captured by King Sandy and the Knights of the Round Towel in order for Sandy to force Kuki to marry him. During this encounter, Sandy inadvertently falls in love with Mushi and abandons his pursuit of Numbuh 3.