Yōko Katori serves as a B-rank All-Rounder and captain of Katori Squad within Border. Her childhood friendship with neighbor Hana Somei evolved into a partnership when Somei became her squad operator. Before the First Large-Scale Invasion, Katori embraced a carefree lifestyle dominated by video games, relying on innate intelligence over effort in academics or pursuits. The invasion destroyed her home, burying her under debris until Somei rescued her. While Katori’s family survived, Somei’s perished, driving both to join Border. Katori envisioned their combined talents—her self-proclaimed genius and Somei’s diligence—effortlessly elevating them to prominence.

Katori’s personality orbits her "genius" self-image, breeding arrogance and laziness. She historically attained high results with minimal exertion but faced a confidence crisis encountering stronger Border agents. This spurred repeated combat-class switches, outwardly dismissed as boredom while internally she wrestled with an "insurmountable wall" separating her from elites. Defeats by squads like Nasu and Tamakoma Second ignited self-loathing and apathy toward her team’s rank. She deflects criticism by flaunting superiority and openly mocks platitudes about hard work or dreams. Envy of others’ perseverance sometimes pierces her facade, notably when losses intensified self-hatred and nearly prompted her Border resignation.

In combat, disillusionment initially led her to dismiss strategy and opponent analysis. Overconfidence bred recklessness, such as solo-charging Yūma Kuga against squad advice during Rank Wars. Yet she displayed tactical cunning, feigning rage to mislead foes and wielding psychological warfare to unsettle adversaries like Osamu Mikumo. Her All-Rounder versatility anchors her fighting style, though team coordination suffered from dismissiveness toward teammates Rokurō Wakamura and Yūta Miura. A pivotal Rank Wars moment against Tamakoma Second saw Mikumo exploit her aggression, baiting her into a last-second wire trap as he bailed out, enabling Kuga’s victory.

Her relationships reflect complexity. She deeply values Somei, joining Border partly to remain with her post-invasion. Their dynamic merges mutual reliance with friction, as Somei’s diligence clashes with Katori’s resistance to effort. Within her squad, she frequently belittles Wakamura’s combat skills and rejects his leadership, while Miura occasionally glimpses her insecurities. Katori resents elite agents she cannot equal and scorns idealists like Mikumo. Outside battle, she savors video games, senbei rice crackers, and winning. She prizes popularity and aesthetics, even copying uniform designs from peers to attract admirers.