Run Momoki is a second-year high school student sporting light ochre hair styled in a distinctive parakeet-like fashion. Her enduring childhood friendship with Tooru Ichii continues into high school, where she also forms strong bonds with classmates Yuko Nishi and Nagisa Tennoji. Run's sociable nature and optimistic outlook make her popular, including with male classmates, though Tooru frequently steps in protectively during these interactions.

Run exhibits notable forgetfulness and absentmindedness in daily life, often requiring friends' help to prevent mishaps or self-injury. Examples include forgetting essential items like underwear or school materials. Her mornings are especially challenging due to extreme difficulty waking, frequently needing Tooru's assistance to arrive at school prepared. Even when awake, she remains semi-drowsy, leading to unreadable class notes or missed instructions.

Despite this, she consistently shows kindness and offers emotional support to her friends. Her naive perspective helps her find positivity in adverse situations. She actively participates in group activities such as New Year shrine visits, pancake outings, and caring for Yuuko during an illness, as depicted in the OVA "A Channel + smile". Academically, she struggles with focus during classes, often appearing sleepy, but is not characterized as unintelligent.

A distinctive habit involves styling her bangs to expose her forehead, which inadvertently draws attention from the school health teacher, Sachiyo Sato. Run remains unfazed by this attention, though it concerns her friends. Her preferences include a strong partiality for grilled meat, especially Kobe beef, reflected in her customized ringtone.