Ōi, the fourth Kuma-class light cruiser originally built in Kōbe, began service as a Navy School practice vessel before undergoing significant conversion into a specialized torpedo cruiser. Multiple remodels reflect this tactical shift. Her initial form offers moderate combat ability. After her first remodel at level 10, she reclassifies as a torpedo cruiser, gaining substantially enhanced torpedo firepower (80 base) alongside higher ammunition consumption. A second remodel at level 50 unlocks her Kai Ni configuration, improving durability, evasion, and torpedo power (90 base, 139 max) while adding an equipment slot. This version carries a 12.7cm twin high-angle gun mount, a 61cm quintuple oxygen torpedo mount, and a Type 21 air radar. Her personality centers on fierce dedication and a deeply possessive, protective bond with her sister ship Kitakami. Dialogue persistently references Kitakami’s whereabouts, expressing acute worry during separation and assuming imminent danger requires her intervention. She questions the Admiral about Kitakami’s location during routine interactions and demands immediate reunions. Derivative media reinforces this attachment through actions like purchasing matching swimsuits or being fed by Kitakami. Though formally polite in introductions, Ōi shifts to impatience or veiled threats when challenged or jealous. Secretary lines interrogate the Admiral’s motives ambiguously, while docking lines include muttered criticisms of command followed by insincere apologies. When married, she explicitly mentions Kitakami during the ceremony and warns the Admiral of sinking them for betrayal. She exhibits distinct fascination with torpedoes, describing oxygen torpedoes as "cold and marvelous" during equipment acquisition and eagerly anticipating their destructive potential in combat. Across official media, including the first anime season, Ōi appears consistently alongside Kitakami, primarily in background or non-combat base scenes. She visually participates in fleet formations and briefings, like the response to the Abyssal threat near the naval district, but avoids frontline action and lacks independent narrative focus. Her role remains supportive, emphasizing traits through proximity to Kitakami rather than solo development. Subsequent seasons, films, OVAs, or spin-offs maintain this portrayal without substantive changes beyond game mechanics and minor anime cameos.