Sachiko Hirokouji, nicknamed "Hiro-chan," resides as a college art student tenant at Coquelicot Manor, the Matsuzaki family's boarding house. She lives alongside Umi Matsuzaki, Umi's younger siblings Sora and Riku, their grandmother Hana, and another tenant, doctor-in-training Miki Hokuto. An average-looking woman in her early thirties, she typically wears dark brown rectangular glasses, an orange uniform, blue jeans, red shoes, and a paint-stained apron over her clothes, styling her hair in a bob cut. Her room remains characteristically messy and chaotic, filled with her paintings and art supplies.

Soft-spoken, Sachiko exhibits traits often associated with artists, including being somewhat awkward, introverted, and eccentric. She keeps nocturnal working habits, frequently requiring Umi's wake-up calls from the kitchen each morning. Despite her artistic preoccupations, she willingly assists Umi with domestic chores, especially cooking, and demonstrates knowledge of the household's organization, such as the location of ingredients stored in a trapdoor. She possesses a substantial appetite, evident during meals and social events like Miki Hokuto's farewell party and Ryoko Matsuzaki's return celebration.

Her primary focus is her artwork, notably a large, nearly abstract painting titled "Poppy Hill" that she works on during the film. Influenced by styles similar to Kandinsky, she uses color expressively to convey emotion within this depiction of Poppy Hill. The only clearly recognizable element in the painting is the flag signal used by Umi for maritime communication, specifically referencing the signal hoisted by Shun Kazama in response to Umi's flags. She is a skillful yet introverted artist who finds solace and expression in her creative work within the bustling boarding house environment.