Maria Tarō Sekiutsu is an illegal immigrant of unspecified nationality, with some sources indicating potential South American origins. She purchased the birth certificate and class registration of a male student named Tarō Sekiutsu—a name derived from the phrase "seki uttaro," meaning "I'll sell my name" or "You sold your own register, didn't you?" This transaction enables her to attend school under his male identity despite being female.

Physically, she possesses dark skin, short black hair, and blue eyes. As the shortest student in Class 2-F, she wears an ill-fitting uniform too small for her frame. Her appearance, combined with extreme poverty, frequently triggers protective instincts in others.

Her personality radiates relentless optimism, immaturity, and innocence. Cultural misunderstandings often provoke wild behavior, offering outsider perspectives on Japanese society. An energetic, childish demeanor includes hoarding discarded food or refuse, reflecting both her fascination with Japan's perceived abundance and her impoverished background.

Notable traits include an aversion to wearing shoes and panties, causing visible discomfort. Most characters call her "Maria" despite her official registration as Tarō, while Kafuka Fuura dubs her "Mataro." The class collectively shields her due to her illegal status and vulnerability, routinely providing material support.

Her story begins in Chapter 9 (Episode 3 of the first anime season), where she assumes Tarō Sekiutsu's identity. The class discovers her background but accepts her, charmed by her demeanor. In Chapter 34, she adopts a "tsukkomi" role—slapping classmates acting as "boke" (eccentric fools)—and frequently delivers punchlines in the final panel of manga chapters.