Itsuki Harune is Aira Harune's younger brother, introduced at age 11 in *Aurora Dream* and aged 14 by *Dear My Future*. He possesses navy blue hair and dark blue eyes.

In *Aurora Dream*, Itsuki holds a secondary role. He displays notable maturity, often acting as the most sensible sibling. He frequently teases Aira about her clumsiness and romantic interactions, especially concerning Shou, yet consistently supports her Prism Star pursuits. Itsuki shoulders family responsibilities, playing with the younger twins Uru and Eru when Aira is unavailable. He actively helps Aira conceal her Prism Star activities from their overprotective father, collaborating with the twins to keep the secret. He demonstrates perceptiveness by identifying and explaining to Aira why Uru and Eru became upset with her comments.

Itsuki's role significantly expands in *Dear My Future*. Now a middle-school student with a more refined appearance, he attracts romantic attention, particularly from Reina Miyama who sustains a crush throughout the series. Despite his earlier perceptiveness, Itsuki remains entirely unaware of her feelings. His entry into Prism Shows occurs unexpectedly after returning a dropped smartphone to Mia Ageha. This leads to an invitation to perform in a "Surprise" themed Prism Show with Prizmmy☆. Though primarily focused on academic studies to achieve his stated goal of becoming a sports doctor, Itsuki experiences surprising thrill and happiness during the performance. His natural athleticism is noted, described as proficient in most sports.

Following his debut, Yong Hwa recruits him to form the male Prism Show unit Love∞Mix. Initially hesitant, Itsuki eventually embraces his role as an idol within the unit, discovering genuine enjoyment in performing Prism Shows. Crucially, this new path does not divert him from his original ambition; he remains steadfast in pursuing a career as a sports doctor. The narrative confirms his eventual achievement of this professional goal in a depicted future scenario.