Atri is the central character of the story, a remarkably advanced robot girl discovered by the protagonist, Natsuki Ikaruga. Her physical appearance is that of a young, delicate girl with long, light-colored hair and large, expressive red eyes. She is typically seen wearing a distinctive, oversized white coat with blue cuffs and a red ribbon, and for a significant part of the narrative, she is notably barefoot.

Her background is intertwined with Natsuki's family. She was originally created as a highly advanced android and served as an assistant to Natsuki's grandmother, a scientist. For reasons tied to a secret plan, she was placed in a special capsule and slept for three years at the bottom of the ocean. When Natsuki retrieves her, she awakens with significant memory loss, unsure of the final command her previous owner left for her. Despite her amnesia, she recognizes Natsuki as her new master and decides to follow his orders.

In terms of personality, Atri is far from the stereotypical, emotionless machine. She is depicted as being highly curious about the world around her and is emotionally expressive, displaying a wide range of feelings from joy to sadness. A defining verbal tic is her frequent, proud declaration, "I'm high-performance!". Her behavior is often naive and mischievous, leading to comedic situations, particularly when she tries to perform household chores with disastrous results, earning her the nickname "useless robot" from Natsuki. Despite her cheerful exterior, her core motivation is a deep-seated desire to fulfill the last, forgotten order given to her by her original master.

Atri's primary role in the story is to be both a companion and a catalyst for the protagonist. She becomes Natsuki's "leg," providing him with physical and emotional support as he struggles with the loss of his own leg and deep-seated trauma. Her presence forces him to reconnect with the world and other people. Her key relationship is, of course, with Natsuki. As they spend the summer together, a deep and complex bond forms between them, blurring the lines between master and property, and evolving into a mutual, though complicated, romantic affection. She also forms important connections with the small community on the island, including Natsuki's childhood friend, Minamo, and the other local children, whom she attends school with.

Her character development is central to the narrative's philosophical questions. Initially, she presents a mostly cheerful and helpful demeanor. However, as the story progresses, Atri grapples with her own identity, questioning whether her feelings for Natsuki and her friends are genuine emotions or simply the result of complex programming simulating human behavior. This internal conflict leads to moments of great vulnerability, where she admits to mimicking emotions because her programming judged it to be the most appropriate response. Her journey is one of seeking to understand the concept of a "heart," a quality she believes she lacks as a robot, and ultimately finding her own answer to what it means to be more than a machine.

As a robot, Atri possesses several notable abilities. She is incredibly strong and durable, with a carbon fiber skull and robust construction. She is also a proficient swimmer, capable of rescuing Natsuki from drowning. However, her abilities are balanced by clear limitations. She requires sleep to rest and consolidate her memories, a process analogous to human dreaming. Most critically, her power source is not infinite; she has a limited operational lifespan after being awakened, a countdown that forms the emotional core of the story's timeline. A seemingly simple but profoundly important object associated with her is a pair of tea-colored leather shoes that Natsuki buys for her. These shoes symbolize his acceptance and need for her, becoming a crucial emotional anchor and a key item in determining the story's outcome.