Naori Shingūji sports brown hair, often in twin pigtails during high school, contrasting with her twin sister Rumi's shorter style. She possesses a curvy figure and is well-endowed. Academically gifted, she achieves high grades, though not as high as Jun Shirosaki's, sharing his fondness for advanced reading material. Her personality is outgoing, tomboyish, and assertive; she speaks her mind openly and defends her stance without hesitation. She naturally integrates otaku media references into conversation.

Born minutes after Rumi, Naori met their childhood friend Jun at age five when their mothers introduced them to the neighboring Shirasaki family. Unlike the shy Rumi who hid, Naori blushed upon seeing Jun but initially looked away. She frequently initiated childhood play sessions with Jun and Rumi. During grade school, she faced bullying from classmate Furose when accused of reading inappropriate material, leading to ongoing conflicts where Naori consistently retaliated verbally until Jun mediated. In middle school, she supported Rumi's secret relationship with Jun, unaware of her own romantic feelings for him. Discovering their relationship, she outwardly expressed pride but privately wept, conflicted by unacknowledged jealousy. After Rumi abruptly ended the relationship, Naori confronted her sister for causing Jun pain, criticizing manipulative behavior.

Following the breakup, Naori began dating Jun after he asked her out during a movie date. She immediately informed Rumi, who pretended support while hiding regret. Naori's romantic approach is direct; she attempted to seduce Jun multiple times, including joining him naked in a bath and pressing against him, though he declined due to unresolved feelings for Rumi. She becomes jealous of other girls interacting with Jun and once joked about contemplating murder. Her relationship with Rumi deteriorates significantly; she lied about losing her virginity to Jun to provoke Rumi, declared she no longer wished to be sisters, and admitted deliberately toying with Rumi's emotions—even attempting to set her up with another boy to eliminate her as a rival. This culminated in physical altercations, including a mutual slap escalating into a brawl halted by Jun's intervention.

Naori is candid about personal matters, like revealing her use of thong underwear. She maintains solid relationships with her parents, though her father occasionally mixed up the twins during childhood. Her friendship with Ririsu Kamedake includes outings like karaoke, but she struggles with Shiena Amamiya's excessive physical affection and boundary disregard despite repeated objections. Conflicts with Rumi stem from deep-seated romantic rivalry and familial loyalty, leading Naori to oscillate between resentment and moments of raw honesty, like falsely claiming intimacy with Jun to force Rumi to confront lingering feelings.

Her development shows a persistent struggle between her assertive exterior and vulnerable insecurities regarding Jun's affections. She remains academically competitive, using test scores to engage Jun's attention, and navigates romantic pursuits with a blend of confidence and manipulation, particularly in undermining Rumi's emotional stability. Complex relationships highlight unresolved tensions between familial duty and personal desire.