Yuuna Okuyama (also spelled Arina Okuyama in some merchandise) is a kindergarten classmate of Souta Hori. She initially bullied Souta over his perceived sister complex before Izumi Miyamura's intervention and kindness led her to become friends with him.

As a child, she has chin-length red hair, lilac eyes, and wears a flower hair clip (a gift from Miyamura during their first meeting) on the left side. She typically wears an orange sweater over a white button-up shirt and a skirt. Maturing into a teenager of average height, her hair grows to mid-back length; she clips her bangs while tying or clipping back the rest, leaving chin-length strands framing her face.

Personality-wise, Yuuna displays greater maturity and cynicism than Souta, frequently scolding him and other classmates for childishness. A persistent trait is her self-consciousness about her forehead, perceived as overly large since childhood and continuing into her teens, though slightly lessened after Miyamura compliments her. Her relationship with Souta evolves from antagonism to genuine friendship, demonstrated by her warm interactions with the Hori family during a Christmas Eve visit. She develops a fondness for Miyamura as an older brother figure due to his initial kindness and similarly regards Kyouko Hori as an older sister.

Her background includes attending kindergarten with Souta. Across media, her core traits of maturity and lingering insecurity remain consistent, with her relationships to the Hori family and Miyamura central to her role.