Gurio Umino is a recurring character who appears as a classmate of the protagonist Usagi Tsukino at Juuban Public Middle School. Typically addressed by his family name, Umino is a fourteen-year-old student known for being highly intelligent and academically gifted, often achieving top scores on examinations. His most distinctive physical trait is his large, round glasses, which are famously depicted with spiral patterns on the lenses. Despite his ordinary and somewhat geeky appearance, the author of the series has indicated that he is actually handsome when his glasses are removed, a fact that the other characters briefly speculate about.

In terms of personality, Umino embodies the archetype of a stereotypical nerd or "know-it-all." He is socially awkward and can come across as arrogant, often boasting about his academic performance without much provocation. He also has a tendency to suddenly appear in unusual places, seemingly out of nowhere, to inject himself into conversations. One of his primary functions within the narrative is to serve as a source of gossip and exposition. He is deeply interested in current events, strange crimes, rumors, and the activities of local heroes, frequently keeping his friends updated on topics such as the vigilante Sailor V, the Sailor Guardians, and new transfer students.

Umino's role in the story is that of a supportive, if occasionally annoying, friend. His motivations often revolve around his personal interests and affections. Early in the series, he has a significant but unrequited crush on Usagi. At one point, he asks Naru Osaka for advice on how to win Usagi over, and Usagi herself agrees to go on a date with him, though this is later revealed to be part of a scheme to spy on other characters rather than genuine romantic interest. After the death of Nephrite, a character with whom Naru had fallen in love, Usagi actively encourages a relationship between Naru and Umino. This relationship deepens significantly after Umino heroically, albeit comically, impersonates Tuxedo Mask to protect Naru. By the second major story arc, they are established as an official couple and are later seen winning an affection contest together. His devotion to Naru becomes a defining trait, and he is shown to be caring and loyal to her.

Umino’s key relationships are primarily with Usagi Tsukino and Naru Osaka. His friendship with Usagi is a core part of his school life, and his dynamic with Naru evolves from a casual friendship into a romantic partnership. Beyond his social circle, Umino has shown a brief vulnerability to the dark forces that threaten the city, as seen when a monster possessed him with dark energy, turning him temporarily evil alongside other students until Sailor Moon intervened. He also demonstrates a knack for research, using the pseudonym Professor Squiggles for exams and knowing the identity of the genius student known as Mercurius.

While Umino does not possess any superpowers or combat abilities, his notable talent lies in his skill for gathering and disseminating information. He is an endless well of knowledge about people and events, often providing the necessary clue or background that helps the main characters understand the strange occurrences in their city. His last regular appearances in the anime occur during the SuperS season, after which his presence in the story gradually fades.