Maria Tarō Sekiutsu is an undocumented immigrant whose exact nationality remains unspecified, though contextual hints like her complexion and homeland descriptions suggest a potential Filipino origin. She secured her Japanese identity by purchasing the birth certificate and school registration of a former male student, resulting in her attending school under the legally male name "Tarō Sekiutsu" despite being biologically female. This name derives from the Japanese phrase "seki-uttarō," translating to "I'll sell my register" or "You sold your register, didn't you?" – a direct nod to the transaction that provided her documents.

Most characters, Maria included, use the name "Maria," while Kafuka Fuura dubs her "Matarō" after a children's book character. Fascinated by Japan's perceived abundance compared to her impoverished past, Maria frequently scavenges discarded food and items from garbage cans or the ground. She resides in an abandoned apartment building alongside numerous other undocumented immigrants.

Maria exhibits a cheerful and mischievous personality, often observing events with an outsider's detached perspective. She typically refers to herself in the third person and tends to deliver the narrative "last word." Her optimism and resilience align closely with Kafuka's worldview, forging a strong bond between them. Despite her generally bright demeanor, Maria occasionally makes disturbingly casual remarks about traumatic experiences like genocide, hinting at a violent or unstable past in her homeland.

Physically, Maria appears younger than her classmates, with her exact age ambiguous; context indicates she is taller than elementary children but shorter than classmate Meru Otonashi. She adamantly refuses to wear shoes or panties, citing discomfort, and maintains a consistently unkempt appearance. This disheveled look, combined with her illegal status and persistent smile, often prompts others to offer her material assistance.

Her interactions extend beyond Kafuka, including frequent collaborations with Chiri Kitsu during the latter's destructive episodes. Maria's background as an undocumented immigrant surfaces in classroom discussions, where she provides firsthand insights into themes of poverty, displacement, and cultural adaptation. Her character development across the series remains minimal, with her core traits of resourcefulness, optimism amidst hardship, and outsider perspective serving as consistent features.