Momoko Sakura, nicknamed Maruko, is a third-grade student living in Shimizu City, Shizuoka Prefecture, Japan, during the mid-1970s. She resides with her parents Hiroshi and Sumire, her older sister Sakiko, and her paternal grandparents Tomozo and Kotake in a modest household reflecting typical lower-middle-class suburban life of the era.

Maruko possesses short black hair featuring seven distinct locks forming her bangs and notably prominent pink cheeks. Her standard outfit includes a white long-sleeved blouse, a red plaid skirt with shoulder straps, white socks, and red shoes, shifting to short sleeves and shorts in warmer seasons.

Cheerful and friendly by nature, Maruko also contends with laziness and forgetfulness, frequently arriving late to school, neglecting homework, and misplacing belongings like her school bag. Math proves particularly challenging, often drawing reprimands from her mother. Despite these flaws, she exhibits curiosity and a well-meaning spirit.

Her passions center on reading manga, drawing, and listening to music, especially performances by her idol singer Momoe on television. These interests fuel her dreams of becoming a mangaka, singer idol, or world traveler. She demonstrates creativity through activities like ventriloquism and collects stamps. Maruko also excels at goldfish scooping, holding a record of 179 fish, and reciting dodoitsu poetry.

Family interactions deeply shape her experiences. She often quarrels with her responsible older sister Sakiko over possessions and behavior, though mutual care exists beneath the surface. Her mother Sumire frequently scolds her for shortcomings but also shows affection through gifts. Her grandfather Tomozo acts as a close confidant who indulges her and shares her playful tendencies. Her social circle includes classmates and friends, with Tamae identified as her best friend.

Within the narrative of "Watashi no Suki na Uta," Maruko undertakes a school assignment to create artwork inspired by her favorite song. She selects "Menkoi Kouma" (Beloved Foal), learned in music class. This project leads her to Shouko Kimura, a university student drawing portraits at a train station while preparing for an art contest. Their initial meeting blossoms into a meaningful friendship, with Shouko valuing Maruko's sincerity and imagination.

Throughout the film, Maruko deepens her grasp of "Menkoi Kouma" through encounters revealing its layered history: originally a wartime composition about a conscripted foal, contrasting with her grandfather's modified childhood version and Shouko's boyfriend's association with newborn foals on his Hokkaido farm. The song's complex past and her bond with Shouko foster Maruko's emotional growth regarding parting and memory.

Maruko perseveres through initial struggles with her artwork. Her friendship with Shouko proves mutually inspirational, motivating their creative journeys. When Shouko faces conflict over pursuing art in Tokyo versus moving to the countryside with her boyfriend, Maruko intervenes, advocating for balancing personal dreams with relationships, leading to their reconciliation.

Her food preferences favor French fries, meat patties, pudding, and curry, while she dislikes silver-skinned sushi and raw tomatoes. She previously disliked natto but overcame this aversion. Her most treasured possession is a teddy bear, and she receives a daily allowance of ¥30.