Naruko Yokoshima teaches and advises the student council at Ōsai Academy, her own alma mater. She recalls her high school years there fondly, noting her characteristic perverted tendencies were already present. Her appearance features neck-length reddish-brown hair, dark brown eyes, and a height roughly equal to or slightly taller than student Takatoshi Tsuda. She possesses conspicuous curves, though acknowledged as less pronounced than student Aria Shichijō's. She typically wears a purple long-sleeved shirt with white sleeves and collar, tight greyish trousers, a loose belt hanging around her hips, and a blue bracelet on her left wrist.

Her personality is overwhelmingly defined by extreme perversion, surpassing other notably lewd characters. She aggressively pursues younger men, including male students, openly discussing sexual topics, specific fetishes, and her use of sex toys. This conduct makes her widely perceived as unreliable and ineffective as an educator. It's suggested she advises the competent Student Council because their capabilities compensate for her deficiencies. However, she demonstrates occasional competence, efficiently organizing a search when student Suzu Hagimura went missing on a school trip to Osaka and managing other crises effectively. She harbors concerns about societal expectations, frequently lamenting her single status as she approaches the culturally significant age of 25 in Japan. She occasionally exhibits a fondness for children.

Her interactions reveal more: she uniquely provokes Shino Amakusa into adopting the "tsukkomi" role in their exchanges, reversing Shino's usual dynamic. She repeatedly attempts to seduce Takatoshi Tsuda, including trapping him in a room, though he consistently evades her. She once threatened him with the loss of his virginity if Suzu wasn't found during the search. Fellow teachers Michishita-sensei and Daimon-sensei actively work to restrain her, horrified to discover her attempting a sexual act on school premises. She forms a friendship with the newer, more sensible teacher Rika Koyama, often socializing and drinking with her after work; their contrasting personalities create a comedic "boke-tsukkomi" dynamic. A notable incident involves a beach playboy ("Nanpa otoko"): initially targeted by him, her aggressive advances frightened him away. Pursuing him onto the ocean, she is strongly implied to have sexually exhausted him. She also confiscates pornographic material from students, only to be discovered reading it herself in the school supplies room.

Beyond her comedic and predatory traits, she possesses surprising athletic prowess, demonstrated when she single-handedly defeated the male members of the school’s basketball team during a cultural festival without significant effort. Her surname, Yokoshima, combines the characters for "horizontal" (横) and "island" (島).