Niika Katori attends the same high school as Win Kirifuda and belongs to his social circle. She radiates a consistently cheerful and caring personality, often expressing concern for Win's well-being and staying near him during daily activities. This proximity becomes especially apparent after Crysta, a transfer student claiming a prior connection to Win, arrives. Niika visibly shows irritation when Crysta tries to approach Win, reflecting her own strong feelings for him.

She nicknames Win "King of Darkness," though the exact reason for this name is undefined. Earlier in their acquaintance, she addressed him more formally as "Win-kun," indicating classroom familiarity before closeness.

Beyond school, Niika is a famous social media influencer. Her status is publicly demonstrated, such as appearing in a commercial displayed on a large building screen. She participates in typical activities with Win and their friends, including evenings in parks and Duel Masters card game sessions.

Physically, Niika sports brown hair in a side ponytail and has yellow eyes. Her typical outfit is a hoodie and skirt. She consistently wears a specific hair ornament and carries a deck case adorned with a small bird accessory. This bird motif connects to her surname "Katori," interpretable as "Fire Bird" (火鳥).

Niika possesses a notable appetite, described as capable of finishing a large bento meal and immediately consuming 5000 yen worth of additional food at a restaurant. She is present among Win's friends during a confrontation with bullies targeting him. Later, walking home alone, she independently encounters the mysterious Boundary Line, a phenomenon dividing the ordinary world from a realm of ruins.

Her name "Niika" originates from the Duel Masters gameplay term "2 or less" (2以下), pronounced 'Niika', relating to cost-reduction mechanics for dragon summoning. This connection serves as a naming origin rather than a direct element of her character's story or abilities.