Sumi Sakurasawa works as a rental girlfriend for the same agency as Chizuru Mizuhara. She initially joined the service to overcome extreme shyness and improve her social skills. Her timid nature often renders her nearly speechless, leading her to communicate through nods, expressions, sign language, or by writing messages on her phone or in the sand.

Sumi possesses reddish-pink hair, often braided on the right side, and matching reddish-pink eyes. Her slender build conceals voluptuous features and an ample bosom. She does not adhere to a consistent style of dress, changing her apparel based on the chosen date.

Profoundly introverted and socially anxious, Sumi tends to overthink situations, engaging in internal monologues where she imagines every possible outcome of her decisions. Despite these challenges, she shows strong determination to change herself and become more outgoing, which drove her to become a rental girlfriend.

A first-year college student when she begins her job, Sumi was inspired to improve herself by the idol group "Glory's." She is a fan of magical girl anime, her favorite color is pink, and she owns a dog named Suzuri, meaning "inkstone."

Sumi’s relationship with Kazuya Kinoshita begins when Chizuru asks him to go on practice dates with her to build her experience. On their first date, though incredibly anxious, Sumi gradually relaxes thanks to Kazuya’s patient and friendly demeanor. An incident at a bowling alley, where Kazuya protects her from catcalling men, strengthens her trust in him. By the end, she feels content and expresses a desire to see him again soon.

During a second date, Sumi assists Kazuya in searching for a birthday gift for Chizuru, taking him to a shopping mall and later to a play where Chizuru performs. Using her notepad, she explains that a gift’s value lies in the emotion behind it rather than its material worth. She later gifts Kazuya a fashionable beanie for his own birthday.

A significant moment in her development occurs during a spontaneous trip to the beach. Though Kazuya rents her to seek advice on cheering up Chizuru, Sumi leads him on an unplanned journey to the seaside. There, she begins breaking out of her shell, writing in the sand that she wants to help those in pain and stating that anyone who tries to cheer her up would make her happy. She advises Kazuya that any effort to make Chizuru feel better would be appreciated. For the first time, she speaks aloud in front of him, marking a milestone in overcoming her social anxiety.

Sumi’s feelings for Kazuya evolve beyond a professional relationship. She begins to view their time together as two friends hanging out rather than a transactional date. At a dolphin show, she whispers that she has a crush on him, though he does not hear her confession, and she suppresses these romantic feelings.

She views Chizuru as a mentor and friend within the agency, greatly admiring her professionalism. Her encounter with Mami Nanami occurs when Mami approaches her during a practice date with Kazuya. Misinterpreting Kazuya’s nervousness, Sumi cuddles up to his arm, causing further complications. Afterward, she worries she failed as a professional girlfriend but is encouraged by Kazuya’s kind words to continue.

Sumi decided to become a rental girlfriend due to a desire to pursue a career as an idol, inspired by the group "Glory's."