Sumire Yamanashi lives at Sunohara Inn, room 202, and serves as the student council vice-president alongside president Yuzu Yukimoto and secretary Yuri Kazami. Standing at 170 cm, she is the tallest of the trio. Her appearance features dark blue or black hair styled in a waist-length ponytail tied on the left side, with bangs swept to the right, and light blue eyes.

Calm and quiet characterize her personality, though she harbors a fondness for cute things and maintains a cool demeanor. Within the student council dynamic, she embodies the "wife" archetype, contrasting with Yuzu's "child" and Yuri's "seductress" roles. Sumire holds romantic feelings for Yuzu, frequently displaying protectiveness and affection that can manifest as jealousy or close physical proximity. Her relationship with fellow resident Aki Shiina is explicitly sibling-like; she refers to herself as his "onee-chan" (older sister figure) and seeks his advice, initially unaware of his true gender.

She shares a close, sometimes competitive camaraderie with the other student council members. Sumire often participates in group activities involving Aki or Ayaka Sunohara, the caretaker. Her interactions typically showcase her composed nature, occasionally punctuated by lighthearted teasing or displays of loyalty towards Yuzu.