Atsuya Baba is a thirty-four-year-old comedian and a central supporting character in the mystery narrative. He is one half of the popular manzai comedy duo Homosapiens, where he performs as the straight man, or tsukkomi, opposite his partner Kensuke Shibagaki, who plays the funny man. The two have been partners since their high school years, a long-standing relationship that forms the bedrock of his professional life.

In terms of personality, Baba is defined by his optimistic, mild-mannered, and genuinely kind nature. His honest and unassuming character is widely appreciated by the public, which has led to a degree of personal popularity that sometimes surpasses that of his duo. He is generous and lacks a strong, selfish ambition for fame, a trait that significantly shapes his motivations and worldview. Baba holds a deep and open admiration for Shibagaki’s comedic talent, consistently crediting his partner as the genuinely funny member of their act and stating a profound commitment to never stand in the way of Shibagaki’s career, even if it means quitting comedy himself.

Baba's role in the story is primarily tied to his interactions outside of his comedy partnership. One of the most significant of these is with Rui Nikaido, a member of the idol group Mystery Kiss. Their relationship begins on a negative note when Nikaido criticizes their comedy on their radio show. However, a chance encounter on a train leads to a deeper conversation where Baba reveals a crucial piece of his personal development. He admits to having felt frustration in the past about younger, less experienced comedians finding success. Crucially, he explains that his perspective has shifted; he now finds genuine happiness in the success of his peers rather than focusing on his own ambition. This honest and mature outlook resonates strongly with Nikaido, prompting them to exchange contact information and develop a supportive friendship. This connection becomes vital when Baba later provides emotional support to Nikaido after she discovers the body of her missing groupmate, Yuki Mitsuya, even though she does not reveal the full, troubling circumstances of the event.

The narrative of the film reconstructs the events of the original series and shows post-series developments. Within this framework, Baba does not receive new background or a distinct arc of change; rather, his character is presented as consistent with his established persona. A notable aspect of his private life is his secret hobby of playing the mobile game Zooden under the username Homosapi Baba. He hides this pastime from Shibagaki, who criticizes it as a distraction from writing comedy material, though he uses his real surname in the game to engage with fans. He also hosts his own segment on the Homosapiens radio show called What Do I Eat for Lunch?, where he dines with random city residents. As with all characters in this story, his appearance as an anthropomorphic animal is a result of the protagonist Hiroshi Odokawa’s visual agnosia, a condition that causes him to perceive all humans as animals; Baba is a human being, and his animal representation is a horse.