Nonoha Itō stands as a central figure throughout the Phi Brain narrative. Introduced as Kaito Daimon's childhood friend and classmate, their bond predates the death of his parents. She demonstrates longstanding care and responsibility, regularly preparing his meals and ensuring he attends school. Physically, she is recognized by her brown hair styled in a split ponytail and layered brown eyes. She initially wears a standard school uniform: a white t-shirt with a red ribbon, a purple skirt, and pink sneakers. Later attire includes a pink sweater over a darker pink shirt with maroon shorts during the Sage Puzzle exploration.

Her personality blends headstrong determination, physical strength, and occasional recklessness. She exhibits fierce protectiveness toward Kaito, often reacting with physical force like kicks or headlocks when frustrated, particularly over criticism of her cooking. Despite this assertiveness, she consistently denies romantic feelings when questioned. However, her actions suggest otherwise—she displays jealousy when other females interact with him and recruits Gammon to monitor such situations, embodying the tsundere archetype. While notably poor at solving puzzles, she compensates with exceptional athletic abilities and a precise photographic memory. Her cooking, especially sweets, receives widespread praise from others but is notoriously feared and criticized by Kaito, who labels them "weapons of mass destruction."

Nonoha's involvement in the main plot begins when she accompanies Kaito to the Sage Puzzle ruins after an invitation from "Minotaur." Ignoring Kaito’s warnings about the deadly labyrinth, she employs an online tactic of keeping one hand on the wall, inadvertently triggering a trapdoor that nearly kills her before Kaito rescues her. Her memory proves vital later when backtracking through the puzzle, and she notices subtle structural inconsistencies aiding progress. This pattern continues across all seasons as she consistently supports Kaito during philosopher’s puzzles deployed by P.O.G. and later the Orpheus Order, despite her limited puzzle-solving skills.

A significant development occurs when she assists kindergarten children during a puzzle challenge, earning her the unsettling title "Nightingale," which she associates with eccentricity. The moniker "Monster Lady" also references her physical prowess. Her role evolves beyond simple support; during Kaito’s hospitalization in one arc, she devises a comedic boke and tsukkomi routine to uplift his spirits. In another instance, her athleticism enables her to swim for six hours to reach a resort during a beach episode after prior commitments delay her. She occasionally becomes entangled in deadly puzzles as a damsel-in-distress, though one occurrence subverts expectations through a gender-role reversal—Kaito wears a dress while rescuing her when she is dressed as a prince.

Her relationships are defined by loyalty and complexity. She shares a deep, platonic-yet-affectionate bond with Kaito, who shields her from harm—such as concealing that their friend Madoka is a P.O.G. agent—though he openly disparages her culinary efforts. Gammon Sakanoue harbors romantic feelings for her, which she either overlooks or exploits, frequently compelling him to accompany her when monitoring Kaito. She uses nicknames like "Ku-kun" for Cubic Galois and maintains protective dynamics with other members of Kaito’s puzzle-solving group.

Trivia notes include her name being a pun on "Tower of Hanoi" ("hanoi no tou" in Japanese, an anagram of "Itou Nonoha"). Her surname combines "well/mine shaft" (井) and "wisteria" (藤). She appears across all anime seasons and the manga *Phi Brain: The Last Puzzle*.