Niina Ōiso has a strong preference for fighting games, which leads her to become somewhat distant and reserved within the Game Club. Her focused interest in a specific genre causes her to interact less frequently with her fellow club members.

Despite her quiet and reclusive nature, Niina is shown to be kind and considerate. This is demonstrated when she gives Keita Amano a souvenir from the club, reflecting a thoughtful gesture following his invitation to join by the other members.

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Niina Ōiso is an attractive teenage girl of average height, with long brown hair that frames her face in soft bangs on either side, and matching brown eyes. She typically wears the standard Otobuki High School uniform, though she adds a personal touch by leaving the top button undone to reveal a black blouse underneath.

According to Karen Tendō, Niina Ōiso initially began playing video games in an effort to “bring her friend back from the dark side.” While the exact meaning of this comment remains unclear, it highlights Niina’s deep involvement with gaming, which has led her to become somewhat socially withdrawn and isolated from her peers.

In the Gamers! light novels, Niina eventually meets Tasuku Uehara and forms a casual friendship with him. She often refers to him as “Fake Umehara,” a playful nickname referencing their first meeting, which involved a match in a fighting game comparing him humorously to professional Japanese gamer Daigo Umehara.

In the series’ DLC content, it is revealed that Niina uses the online handle SALT0519 while playing fighting games. This username is a wordplay on her surname, which can be rearranged to spell the Japanese word for “salt.” She maintains a one-sided rivalry with another online player, Torabasami (real name: Ayumu Kiriya). Although Niina frequently outperforms Ayumu in matches, she does not share his competitive animosity and instead views him as a friend.

After graduating from Otobuki High School, Niina enrolls at the same local college as Gakuto, Ayumu Kiriya, and Ao Saika.

The given name Niina (新那) is composed of the kanji nii (新), meaning “fresh” or “new,” and na (那), which can mean “what” or act as a phonetic element in names. Her surname, Ōiso (大磯), combines ō (大), meaning “big” or “great,” and iso (磯), meaning “beach” or “rocky shore.