Eiko Adachi is a park employee introduced during a critical hiring period to address a severe labor shortage. She has long, straight black hair, which she often accessorizes with a pink headband, and gold-orange eyes that are frequently noted for their calm and gentle appearance.

Her outward personality is defined by a profound sense of kindness and politeness, presenting herself as a well-mannered and unflappable young woman. This composure is remarkable, as she remains completely undisturbed by situations that would typically alarm a normal person, such as the unsettling appearance of a mascot not wearing a magical disguise charm or the chaos erupting around her. However, the core of her character is built on a significant and deliberate misunderstanding: she formerly worked for a talent agency called Quattro Productions that specialized in making videos she refers to as AVs. While the acronym naturally leads everyone to assume she means adult videos, she is in fact referring to Animal Videos. She is genuinely oblivious to the shock her phrasing causes, innocently wondering why people always react so strongly to her former occupation.

Eiko joined the park specifically because, although she enjoyed her previous work, she wanted a job where she could personally see the smiles on her customers' faces in real time rather than through a screen. She is hired as part-time staff and initially works in Moffle's House of Sweets before later being reassigned to Macaron's Music Theater. Her primary motivation throughout the narrative is to be a supportive and reliable friend. She forms a close trio with fellow newcomers Biino Bando and Shiina Chujo, often acting as the calm, steady presence between them. She is notably protective of Shiina, treating her with a kindness that makes Shiina want to call her big sister.

Eikos personal life reveals a much more complex background. She is the daughter of Eizo Adachi, the director of Amagi Hospital, one of the largest hospitals in the region. Despite her wealthy upbringing, she is engaged to a persistent man named Shuichi, whom she regards with visible loathing. Her familys influence becomes a central plot point when a confrontation between Macaron and Shuichi leads to her father threatening to have Macaron fired and the park shut down out of wounded pride. In a desperate attempt to protect her colleagues, Eiko lied to her father, claiming she was in a romantic relationship with Macaron to give her a reason to break off the engagement. This lie, born of shame and a desire to prevent Macarons firing, spirals into a major crisis. She feels deeply responsible for the situation and is so committed to fixing it that she declares she would go to drastic lengths to prevent her father from harming her new family at the park.

Over the course of her time at the park, she develops from a part-timer with an odd past into a more integrated and vital member of the team. Her development highlights her fierce loyalty and willingness to stand up to her powerful family. She is also a key member of the Task Force ABC, an idol group formed with Biino and Shiina. Macaron, a seasoned performer, notes that she is a good singer but feels she is always holding something back, as if restrained. Aside from her unnerving calmness and unexpected talent for singing, her most notable ability is her inadvertent talent for creating bizarre marketing ideas. For a promotional video, she suggested the idea of equestrianism, citing a video of a horse giving birth as an uplifting example, leading to a shockingly graphic image being included in the final park advert.