Ema Mizunami is a first-year student at Motosu High School, the same school attended by the main cast of the series. She first appears during the spring break before her high school entrance, meeting a key character while taking a walk in Fujikawa Park. Her primary hobby and passion is drawing, and she is an aspiring illustrator who works digitally on a tablet. To acquire this tablet, her parents paid for half of it as an entrance gift, while the other half was a loan she committed to repaying herself. This financial responsibility is a major motivation for her actions early in her story.

Ema is characterized by a notably responsible and organized personality. She often acts as a grounded presence among her more relaxed or eccentric friends and acquaintances, frequently helping to keep situations orderly or offering practical support. Her nature is also good-humored, and she is known to make dry jokes. She has a great love for dogs, but due to restrictions where she lives, she cannot own a real one. To compensate, she has created a vivid imaginary dog, which she mentally selects from a menu inspired by classic video games, similar to choosing a weapon.

One of her most defining abilities is her talent for digital illustration, which she practices regularly on her tablet. Beyond her artistic skill, Ema demonstrates a surprising aptitude for logistical planning and organization. When participating in group activities, she often takes charge of practical details, such as making reservations, planning routes, and taking initiative when others are less focused. This managerial skill leads her seniors to praise her as a highly capable underclassman.

Ema's role in the story begins before her school life starts. By chance, she encounters Ena Saitō and her dog Chikuwa napping in the park. Inspired by the peaceful scene, she draws a sketch of them, which serves as the catalyst for their new friendship. After entering Motosu High School, she reconnects with Ena. While exploring the school for a quiet place to draw, she ends up in the library, where she has an initial, intimidating encounter with Rin Shima. A funny message from Ena causes Ema to laugh out loud, breaking the tension and allowing her to get to know Rin as well. She finds the older students' personalities unique and refers to them as her strange seniors.

To repay the debt for her tablet, she begins working part-time at the Caribou outdoor goods store near Minobu Station. She gets this job on the recommendation of Rin Shima. At Caribou, she works alongside a colleague named Ibigawa, who later provides her with valuable recommendations for camping locations. Although she is not a member of any school club, finding that neither the art nor manga club suits her interests, she becomes involved with the outdoor activities circle through her friendships. When Nadeshiko Kagamihara plans a camping trip during Golden Week, Ema joins in, suggesting a trip to Lake Nozori in Gunma based on her coworker's advice. During this trip to Karuizawa and Gunma, she fully demonstrates her reliability. While her older friends sleep on the train, she takes scenic photos, and she proactively books a taxi in advance, naturally assuming the role of the group's effective guide. Through these experiences, her interest in the outdoors grows, leading her to borrow camping equipment from her employer for future excursions.

Her key relationships are central to her development. She shares a close friendship with Mei Nakatsugawa, a friend from middle school. Her new friendship with Ena Saitō is her primary connection to the main group. Through Ena and Rin, she also gets to know Nadeshiko Kagamihara, whose boundless energy she finds both admirable and a bit strange. Her part-time job at Caribou also adds an important adult connection to her circle as she interacts with the shop staff. Ema Mizunami thus grows from an incoming student with a personal hobby into a valued, reliable, and well-integrated member of the community, beloved for her competence and good-natured humor.