Ami Matsuzaki is a five-year-old girl who recently moved to a new home with her parents, including her mother Ayako Matsuzaki. She attends kindergarten with her human friend Kumiko Takeda, known as Ku-chan. Ami discovered a doorway on her bedroom floor serving as a portal to Aniyoko, an alternate dimension inhabited by anthropomorphic stuffed animal-like beings. This discovery resulted in frequent visits from three primary Aniyoko residents: Iyo, a rabbit-like creature; Kenta, a bear-like creature; and Issa, a panda-like creature. Collectively called the Happy Aniyoko Trio, these beings often engage in unpredictable antics testing Ami's patience. She gradually becomes accustomed to their presence and shares aspects of her daily life with them.

Ami possesses an intelligent, grounded, and imaginative personality, yet she is easily flustered by the Aniyoko Trio's behavior. Despite her young age, she displays maturity beyond her years, acting as the group's most rational member. She keeps the Aniyoko doorway and its inhabitants a secret, especially from her parents, who remain unaware of the otherworldly activities in her room. However, she confides in Ku-chan, who gains the ability to perceive the trio as living beings. The Aniyoko doorway influences Ami's bedroom environment, enabling unexpected journeys to locations like jungles, forests, or dungeons without physically leaving the room.

Born on December 15 under the Sagittarius sign, Ami has blood type B. She consistently wears the same outfit, highlighted when the trio discovers multiple identical garments during a laundry incident. Although she could traverse the doorway to visit Aniyoko, she refrains throughout documented events, observing only darkness when looking through. Her interactions primarily involve managing the trio's disruptive behavior while maintaining ordinary routines like homework or birthday celebrations, with her age remaining static at five years old across the timeline.