Sachiko Hirokōji, a college art student in her early thirties, rents a room at Coquelicot Manor, a boarding house overlooking Yokohama's harbor. Her space brims with canvases, art supplies, and vibrant colors, creating a noticeably chaotic and cluttered environment compared to the other rooms. Soft-spoken and somewhat eccentric, she embodies traits often linked to artists, including social awkwardness and unconventional behavior. Prone to working late into the night on her paintings, Sachiko frequently appears sleepy or lethargic, leading to moments where she dozes off during daytime activities like helping with cooking. Other residents often wake her in the mornings.

Despite her sleepiness, Sachiko assists with household chores, particularly meal preparation, knowing the kitchen layout well and even accessing vegetables through a trapdoor. She possesses a notably hearty appetite, seen enthusiastically enjoying food, such as eating beef jerky alongside Riku Matsuzaki during a farewell party. Casual remarks or questions sometimes elude her, like asking if beef jerky is made from pork.

Her main artistic endeavor is a painting titled "Poppy Hill," depicting the hill overlooking the harbor and featuring the recognizable flag signal Umi Matsuzaki uses to communicate with ships. Umi views this significant painting while checking on Sachiko. Sachiko interacts with the Matsuzaki family, forming an incidental connection with Riku through shared moments like eating. She wears practical, paint-stained clothing: typically an orange uniform, blue jeans, red shoes, and a messy apron over her bob-cut black hair, complemented by dark brown rectangular glasses.

Her surname, Hirokōji, translates to "wide alley," while her given name, Sachiko, combines "happiness" or "good luck" with the suffix for "child." Within the boarding house, she is occasionally called "Hiro-chan."