Itsuki Harune is introduced as the younger brother of the main protagonist Aira Harune, residing with his family in Yokohama, Japan. At the start of the series, he is eleven years old and the second oldest of four siblings, having two younger twin sisters, Eru and Uru. He is characterized by his navy blue hair and dark blue eyes, and as the story progresses into the sequel series, he has grown into a fourteen-year-old middle school student.

In terms of personality, Itsuki stands out as the most sensible and down-to-earth member of the Harune siblings. He displays a level of maturity that often surpasses his older sister, and he is known for his calm, collected, and somewhat sarcastic demeanor. He frequently teases Aira about her clumsiness and her awkward romantic interactions with her peer Shou, but this teasing is never mean-spirited. Beneath his cool exterior, he is deeply supportive of his sister and her dreams. When Aira is busy with her Prism Show activities or other work, Itsuki reliably takes on the responsibility of caring for and playing with the younger twin sisters. He is also perceptive, showing an ability to understand the emotions of those around him, such as figuring out why his younger sisters became upset with Aira. A unique personal interest he possesses, at least during his childhood, is a passion for enka, a genre of traditional Japanese music.

A key part of Itsuki's early role in the story is his function as an ally and secret-keeper for Aira. Their father Hiroshi disapproves of Prism Shows, so Itsuki works together with the twins to help Aira conceal her activities as a Prism Star. This requires him to be responsible and quick-thinking, further reinforcing his role as the most reliable sibling.

As the narrative moves into Dear My Future, Itsuki grows into a handsome and refined young man, attracting the immediate romantic attention of the Prism Star Reina Miyama, who nicknames him the Rose Prince. Despite his usual perceptiveness regarding his sister's love life, he remains entirely oblivious to Reina's feelings for him, at least initially. His own career path takes an unexpected turn when he returns a dropped smartphone to Mia Ageha, a member of the idol unit Prizmmy. This encounter leads to an invitation to participate in a Surprise-themed Prism Show. Though his primary focus has always been on his academic studies, with a firm and consistent goal of becoming a sports doctor in the future, Itsuki is surprised to discover the genuine thrill and happiness that performing in a Prism Show brings him. His natural athletic ability, noted as being proficient in most sports, allows him to adapt to the physical demands of the show with ease. The Korean Prism Star Yong Hwa recognizes his potential and, despite Itsuki's initial reluctance, recruits him to form the male Prism Show unit Love Mix. While he embraces this new path as an idol and finds joy in performing, he never abandons his original ambition, continuing to attend preparatory school alongside his entertainment career. The narrative confirms that in the future, he successfully achieves his long-held dream of becoming a sports doctor.