Sakura Adachi begins high school as an intensely introverted and socially isolated student, perceived as cold and aloof by classmates and family, leading to mutual alienation. Her home life lacked warmth, exemplified by learning to suppress her desires, like wanting an aquarium souvenir. This background hindered her ability to form connections or express emotions.

Physically, Adachi has short black hair and light gray/bluish eyes, standing taller than Shimamura. She later grows out her hair, achieving a more mature look. Working part-time at a Chinese restaurant requires her to wear a traditional dress exposing her thighs; she eventually buys one personally because Shimamura enjoys seeing her in it.

Adachi's personality blends social anxiety with deep emotional intensity. She maintains a "resting bitch face" but is internally laid-back, though prone to silent anger. Befriending Shimamura sparks rapid attachment and unacknowledged romantic feelings, including dreams about kissing her. Adachi experiences strong jealousy when Shimamura interacts with others, especially Tarumi, leading to possessiveness. She recognizes this jealousy is problematic but struggles to control it.

Her development focuses on gradual openness. She plans intricate, indirect methods to express feelings, like orchestrating a Valentine's chocolate exchange. After seeing Shimamura with Tarumi at a festival, Adachi delivers a lengthy, anguished phone confession misinterpreted by Shimamura as annoyance. She later formally confesses at a summer festival, entering a relationship with Shimamura. This bond motivates Adachi to improve herself. Socializing remains challenging, but she attempts connecting with Shimamura's similarly introverted sister and becomes more considerate, texting before calling to respect boundaries.

As an adult, Adachi lives with Shimamura. Her social circle expands slightly, and she appears calmer, though Shimamura's teasing still easily flusters her. She reflects self-critically on her past self-isolation, acknowledging her role in estranging others. Core traits like introversion and deep devotion to Shimamura persist.