Izuko Gaen is a middle-aged woman commanding a network of oddity specialists, largely former members of her college Occult Research Club. She possesses wavy long brown hair, orange eyes, and favors hip-hop-inspired attire: a loose light green shirt over a black sweatshirt, maroon pants, running shoes, a white baseball cap, a smiley necklace, and orange headphones. A signature rainbow cloud swirl manifests in her background, symbolizing her mastery over the full spectrum of information; this phenomenon vanishes when her plans fail, exposing vulnerability. Her defining trait is the claim to "know everything," a curse acquired during the creation of the artificial oddity Yotsugi Ononoki. This grants her exhaustive knowledge of past, present, and future, yet she strategically leverages what others don't know to manipulate events. She starkly contrasts Tsubasa Hanekawa, whose humility about knowledge masks insecurity. Gaen extracts "friendship" as payment for aid, building a network of owed favors to exert indirect influence. Her personality embodies strategic dominance and control, often using intellect to dismantle confidence. Gaen wields two specialized swords: Kokorowatari, lethal to any supernatural entity with a single scratch, and Yumewatari, which reverses damage inflicted by Kokorowatari, including resurrection. She interacts with non-physical entities like the ghost Mayoi Hachikuji and demonstrates combat skill sufficient to overpower enhanced individuals like Koyomi Araragi. Her involvement spans critical events. In Shinobu Time, she advises Mayoi Hachikuji and Koyomi Araragi on evading a primordial "darkness," concluding Mayoi must revert to a lost cow or pass on; Mayoi chooses the latter. During Tsubasa Tiger, she confronts Hanekawa about her limitations while coordinating with vampire hunter Episode, explicitly excluding Yozuru Kagenui. In Hitagi End, she attempts to bribe Deishuu Kaiki to abandon his mission involving Nadeko Sengoku, though Kaiki deceives her. She later orchestrates the Ougi Oshino confrontation in Ougi Dark, revealing Ougi as a manifestation of Araragi's insecurities and positioning the revived Mayoi as god of North Shirahebi Shrine to stabilize her existence. In Koyomi Dead, Gaen's schemes collapse under Ougi's interference. In desperation, she kills Araragi, sending him to hell and stripping away her signature rainbow backdrop, exposing her fallibility. She later revives Araragi in May Hell under threat from Shinobu, approving his unpredictable decision to resurrect Mayoi. Following the Ougi conflict, she complies with Shinobu's request to depower her and return her to Araragi's shadow. Gaen is Suruga Kanbaru's aunt and sister to Tooe Gaen, linking her to the series' supernatural lineage. Her actions consistently reflect a broader agenda to manage oddity threats through calculated alliances and information control, cementing her role as a pivotal, enigmatic force.