Daichi Tachiaki, a high school student, was initially branded a delinquent due to his irritable demeanor and violent outbursts. These stemmed directly from chronic nightmares causing severe sleep deprivation, disrupting his daily life and alienating classmates who avoided him. Sora Kashiwagi broke this isolation, approaching Daichi without prejudice. Sora's gift—a drawing of a dream-eating baku—precipitated Daichi's encounter with the real creature, Mukumuku. Consuming Daichi's nightmares, Mukumuku granted him restful sleep for the first time in years, triggering a profound physical and behavioral shift. His appearance transformed from intimidatingly disheveled to noticeably calmer and approachable, startling his peers.

Post-recovery revealed Daichi's true nature: fundamentally kind, well-mannered, and deeply concerned about burdening others. He harbored guilt over past actions, particularly nearly striking Motegi Asa during a sleep-deprived episode. Once rested, he offered a prompt apology, demonstrating his sense of responsibility. Despite his tall stature, Daichi displayed timidity, especially toward supernatural entities; he initially feared Mukumuku might harm him before accepting the creature's vital role in his healing.

Daichi's bond with Mukumuku became central to his life. He expressed profound gratitude for its nightmare-eating ability resolving his insomnia. Though occasionally disheartened by Mukumuku's perceived aloofness, Daichi cherished their companionship. He named the creature Mukumuku after rejecting friends' suggestions of "tough-sounding" names inspired by ancient samurai lords, which they felt mismatched its gentle nature.

His friendships flourished after his recovery. Daichi regarded Sora as pivotal for offering unwavering support. Intimidated initially by Tazuki Kamiya's assertiveness, Daichi formed a genuine friendship with him—facilitated by Mii-kun—and addressed him as "Kami-kun." His bond with Motegi Asa ("Acchan") strengthened significantly after she forgave his earlier aggression; Daichi admired her forgiving nature. As a twelfth grader, Daichi is the oldest among his core friend group of eleventh graders.

While Daichi's background beyond his sleep issues remains unexplored, his development emphasizes overcoming adversity through support and compassion. His journey highlights reconciling with past actions and embracing a peaceful life alongside Mukumuku and his friends.