Katie, the younger sister of Lynn, is the first-person narrator and protagonist of Kira-Kira. When the story begins, she is almost five years old. Near the end of the narrative, she celebrates her 12th birthday. A person of Japanese descent, as are all the main characters, Katie has straight dark hair, and darker eyes and skin than the novel’s white characters. Katie is curious, outspoken, guileless, and loyal to her family—particularly to Lynn. When Katie does not understand something, she asks someone she views as an expert or, as the story progresses, investigates for herself. Thus, she asks a woman who inoculates newborn chicks if the needle hurts them. However, when told that Lynn has lymphoma, Katie searches for information until she discovers her sister suffers with a potentially terminal disease.

Katie is mischievous and boisterous, though seldom out of control. She is often exuberant, as when playing with her cousins or dancing with her friend Silly. Katie sees herself as naughty and hard-headed, though she desires to please her parents. Thus, she wrestles with whether she is a good person. Reflecting on the ranking of bad behavior established by her mother—hitting is the worst, stealing is next, and lying is third worst—Katie acknowledges she has done all three before turning 12.