Mari is introduced as a second-year student in the Passenger Transport Department and a classmate of Naoto Takayama. Standing 165 cm (5’5”) tall with blood type B, she is characterized by her enthusiasm for trains, often carrying a pink digital voice recorder to capture train sounds while traveling across the country. Her career goal is to become a dining car attendant, which leads her to participate in the on-the-job training (OJT) program with Japan Shokudō, an in-car service company, where she later receives an unofficial job offer. In Volume 2 of the light novel, Mari becomes involved in an incident while working as a dining car attendant aboard Hokutosei No. 3.

In the anime adaptation, Mari’s knowledge of Naoto is portrayed as surprisingly detailed, a fact that arouses jealousy among the other female characters. When she realizes that Naoto has attracted the interest of multiple girls, Mari herself becomes the third heroine to develop feelings of jealousy, further complicating the series’ romantic dynamics.

Mari is depicted with short orange hair and light brown eyes, giving her a lively and approachable appearance. A distinctive feature of her design is the headphones she often wears around her neck, symbolizing her passion for recording and listening to train sounds.

Mari is characterized by her cheerful and outgoing nature, which allows her to make friends with ease. She enjoys excitement and has a lively disposition, often bringing energy to the group. In addition to her friendly personality, she has a strong interest in music and possesses an excellent sense of hearing, a trait that complements her hobby of recording train sounds.

Naoto Takayama
Mari and Naoto are old friends from their school days, and their familiarity is reflected in her ease and openness around him. She appears to have harbored a crush on him since their earlier years, a detail that later causes tension with the other girls who show interest in Naoto.

Noa Kashima
Mari views Noa as both a friend and a romantic rival. While the two maintain a generally positive relationship, Mari is aware of Noa’s interest in Naoto, which places them in subtle competition. Despite this rivalry, they continue to cooperate and interact amicably.

Mari’s given name is written in hiragana (まり), which has no inherent meaning in this form. When expressed in kanji, however, it can take on different interpretations, such as 真理, meaning “truth” or “verity,” or 真里, which combines “real, genuine” (真, ma) with “village” (里, ri). Her surname, Sasshō (札沼), is composed of 札 (satsu), meaning “tag,” “placard,” or “counter for bonds,” and 沼 (shō), meaning “swamp,” “pond,” or “lake.” Together, the name evokes imagery of both authenticity and nature.