Naruko Yokoshima teaches and advises the student council at Ōsai Academy, her alma mater. She possesses reddish-brown neck-length hair, dark brown eyes, and stands roughly as tall as or slightly taller than student Takatoshi Tsuda. Her typical outfit includes a purple long-sleeved shirt with white sleeves and collar, tight greyish trousers, a loose belt worn low on her hips, and a single blue bracelet on her left wrist. She takes pride in her physical appearance, particularly her bust, though acknowledging it is less prominent than student Aria Shichijō's.

Her personality features an extreme preoccupation with sexual matters, exceeding even other characters known for such tendencies. She openly discusses explicit topics, including fetishes like bondage and outdoor sex, and aggressively pursues younger men, including male students. This contributes to her reputation as an unreliable educator, with suggestions her student council advisor role resulted from the council's exceptional competence compensating for her flaws. Despite this, she demonstrates unexpected capability during emergencies, efficiently organizing searches when student Suzu Hagimura went missing during a school trip to Osaka. She expresses recurring concern about societal expectations for marriage, particularly referencing age 25 as a cultural milestone in Japan, and shows occasional fondness for children.

Interpersonally, she uniquely influences student council president Shino Amakusa, often forcing Shino into the role of the straight-man contrary to her usual demeanor. Her interactions with Takatoshi Tsuda involve repeated, unsuccessful attempts to initiate physical encounters, including locking him in a room, consistently rebuffed by Tsuda. She threatens him regarding his virginity during tense situations like the search for Suzu. Among colleagues, music teacher Michishita and physical education teacher Daimon actively monitor her behavior during school trips to prevent misconduct. She later forms a frequent companionship with fellow teacher Rika Koyama, creating a comedic contrast between her extreme attitudes and Koyama's conventional sensibilities during after-school socializing.

Her background includes athletic prowess, evidenced by effortlessly defeating male basketball players during a school festival. Her surname "Yokoshima" combines the characters for "horizontal" (横) and "island" (島). As an Ōsai graduate, she maintained her characteristic personality during her own high school years and remains in contact with former classmates.