Peta is Null's older sister, initially believed to have perished in a truck accident. She possesses long light purple hair styled in twin tails and typically wears a purple blazer with a green skirt, accented by a large red ribbon on her chest.

A twist reveals the accident actually involved Null, struck while chasing after Peta to return forgotten items. Following this, Peta created a virtual world to communicate with Null during her younger sister's unconsciousness and recovery, serving as a rehabilitation environment and explaining Null's survival. The fate of their parents is not detailed.

Within this virtual world, Peta's presence manifests as "Peta Robo," a robot version built by Null's genius intellect to emulate her sister's characteristics and personality, sharing the same appearance. Peta Robo's primary functions are caring for Null and ensuring her well-being: ensuring she attends school, providing nutritious meals despite Null's dislike, acting in her best interests, and even accompanying her to space. Her programmed nature sometimes leads to robotic misunderstandings or literal interpretations of situations, resulting in comedic interactions. Though occasionally forceful, her actions stem from a deep-rooted desire to protect and support Null, reflecting the human Peta's genuine concern.

The relationship between Peta and Null remains central, with both versions demonstrating unwavering dedication to Null's welfare. After Null awakens, the sisters reunite in reality, and Null resumes her normal life, including returning to school.