Shuuichirou Sawaki is the homeroom teacher for Ai Ohto and Koito Nagase, and the art club advisor. He is also Momoe Sawaki's uncle. Physically, he is a tall, handsome man with lightly tan skin, dark brown hair swept to the left, and a distinctive mole under his left eye. He consistently wears a bird-shaped brooch on his collar.

Sawaki regularly visits Ai Ohto's home, explaining it is convenient on his route home. Ai witnesses Koito Nagase crying while clinging to Sawaki in an empty classroom, though the context remains ambiguous. Later, during dinner at the Ohto residence, Sawaki and Ai's mother, Tae Ohto, announce they are dating, seeking Ai's comfort.

As a painter, Sawaki gains recognition after winning an award. He hosts a solo exhibition titled "Latent Heat," revealing a painting of an aged-up Ai Ohto surrounded by red and white camellia flowers. He states this depicts Ai's future potential, comparing her to her mother's beauty and reaffirming his love for Tae. At the exhibition, Ai confronts him about Koito's death.

An alternate version of Sawaki appears in a dream world as a Wonder Killer. This entity manifests as a large, floating, pale, and wrinkly head with four paint tubes protruding beneath it, often smiling. In human form, it mimics Sawaki's appearance but exhibits exaggerated body language and expressions. This manifestation stems from Parallel Ai's doubts about Sawaki.

Floral elements in scenes involving Sawaki carry symbolic meanings. Purple daisies or asters on his desk symbolize patience, fascination, or faith in Victorian and Japanese flower languages. In his painting, white camellias represent waiting or divinity, while red camellias signify romantic love or desire. Visual techniques, such as shots emphasizing him looming over Ai or fragmented compositions, contribute to interpretations of his motivations and ethical boundaries.

The circumstances of Koito Nagase's death are contested. Rika Kawai suspects Sawaki's involvement, theorizing he drove Koito to suicide, though lacking evidence. Momoe Sawaki defends him against these accusations. A later revelation suggests Koito died accidentally after threatening to accuse Sawaki of misconduct, though this is met with skepticism. Sawaki maintains an outwardly calm and polite demeanor when confronted, never explicitly confirming or denying the allegations.