Sonoa Mitsui enters the narrative after Tsukishiro Yuu rescues her from persistent bar harassment. This encounter sparks an emotional attachment, leading Yuu to bring the stranded Sonoa to Tachibanakan dormitory after she misses her last train. Sonoa decides to stay as a guest, expressing immediate fondness for the environment, which she feels matches Yuu's refined atmosphere. Her attachment manifests as consistent romantic interest, particularly noticeable when intoxicated. During these episodes, she perceives Yuu as her "fated prince," embracing her arm, pressing her breasts against her, expressing a desire to travel together to a "garden of white lilies," and declaring her intention to live happily with Yuu at Tachibanakan.

Sonoa exhibits a confident and serious personality, though moments of shyness emerge, such as feeling embarrassed after wearing a sexy nurse costume or after accidentally falling onto Natsuno Hanabi and touching her breasts while nursing her during an illness. She develops a close, sisterly dynamic with Hanabi, requesting to be called by her first name and blushing during their interactions. Her boldness is tempered in specific situations, like declining a separate futon to sleep beside Yuu, only to become flustered the next day when Yuu references their nighttime proximity. During group events, such as a water gun survival game, she initially believes Yuu is advancing to protect her, only to be targeted by Yuu's special move. When Hanabi prepares a futon for her in the guest room during another drunken episode, Sonoa excitedly embraces Yuu, reinforcing her view of Tachibanakan as a wonderful place because of Yuu's presence. Her eyes form heart shapes when Yuu escorts her to their room, underscoring her sustained romantic focus on Yuu.