Nyaruko appears as a silver-haired, golden-eyed teenage girl, concealing her true nature as a terrifying, chaotic extraterrestrial entity incomprehensible to humans. Adopting the name "Nyaruko" on Earth, she serves as a "Space Defense Agency Official" tasked with protecting Mahiro Yasaka from alien threats. This duty swiftly transforms into an intense, persistent romantic obsession with Mahiro, whom she calls "Nyarko-san." She relentlessly pursues him with overwhelming affection, frequently disregarding personal boundaries.

Her personality radiates extreme hyperactivity, boundless optimism, and a cheerful, often manic demeanor. This cheerfulness can instantly shift to violent aggression when angered or when Mahiro is endangered, often manifesting in destructive force using makeshift weapons like forks. Despite her chaotic actions, she possesses genuine care and fierce protectiveness towards Mahiro and her friends.

Nyaruko wields immense superhuman strength, speed, durability, rapid regeneration, and energy projection. She can survive in space, withstand severe injuries, and partially or fully transform into her monstrous true form for greater power. She develops a deep fascination with Earth's pop culture, particularly anime, manga, video games, and otaku subculture, which heavily influences her speech patterns filled with slang, references, and exaggerated romantic comedy declarations.

She forms close friendships with fellow extraterrestrials on Earth, like Kuuko and Hasuta, frequently engaging in chaotic adventures. Interactions with Mahiro and human friends spark subtle shifts; while her core exuberance and obsession remain, moments of genuine emotional vulnerability, jealousy, and a deeper grasp of human feelings emerge. She grows more invested in Earth life and her relationships, balancing her cosmic origins with her adopted human identity. Her dedication to protecting Mahiro and her friends evolves from a duty into an absolute, deeply personal commitment.