Himawari Nohara is Shinnosuke Nohara's infant sister within the film's narrative, residing in Kasukabe with parents Hiroshi and Misae. At the initial 20th Century Expo visit, she displays disinterest alongside Shinnosuke, contrasting the nostalgic adult attendees. When adults succumb to the expo's influence, reverting to childlike irresponsibility, Himawari becomes directly affected. Shinnosuke takes her from home after witnessing their parents' transformation, forcing her into the crisis as society collapses and adults vanish into a recreated 20th century.

She flees with Shinnosuke and his friends—Kazama, Nene, Masao, Bo, and dog Shiro—taking refuge in a department store to evade capture. Following discovery and a bus escape attempt, she survives the crash at the "Adult Empire" complex alongside Shinnosuke and Shiro after their friends' capture. Dependent on her brother for protection, she witnesses Shinnosuke using Hiroshi's foot odor within his childhood memory sequence, countering the nostalgic influence. This action near Himawari helps Hiroshi regain his senses and recall his family.

She remains with the reunited family confronting antagonists Ken and Chaco at the source of the 20th-century influence. Hiroshi, Misae, and Shiro create diversions enabling Shinnosuke's attempt to halt the machine. After the villains' defeat and change of heart, she returns safely to Kasukabe with her family.