Karen Tendo is a second-year student at Otobuki Academy and serves as the president of the school's video game club. She is widely regarded as the most beautiful and admired student on campus, an idol-like figure known for her academic excellence, athletic ability, and seemingly effortless grace. Her appearance is characterized by long blonde hair and green eyes, and she wears the standard female school uniform with a dark blue sweater vest and a black ribbon in her hair. She maintains a strict daily routine that includes morning jogs and completing her homework early, and she makes a point of personally and respectfully rejecting the many romantic confessions she receives.

Behind her perfect public persona, Karen Tendou is also a deeply passionate and competitive gamer. She took it upon herself to revive the game club after its senior members left, recruiting members who share her dedication to competition and improving their skills, while rejecting those whose interest is only in dating her. Her life takes an unexpected turn when she invites the solitary gamer Keita Amano to join the club. When he politely declines her offer not once but twice, citing his preference for playing games for fun rather than for competition, she is shocked and deeply hurt, having rarely faced rejection of any kind. Rather than give up, she begins following him in an attempt to change his mind.

Her feelings transform into a genuine romantic crush after she overhears Keita passionately defend her and her club against a classmate's criticism, listing all of her admirable qualities. This marks a dramatic turning point in her life. The composed and organized school idol becomes flustered, nervous, and prone to wild romantic fantasies whenever she is around Keita. Her carefully managed daily routine crumbles as she finds herself unable to concentrate on her games or her schoolwork, with her thoughts constantly drifting to him. This emotional turmoil leads to a gaming slump and a series of personal embarrassments, a stark contrast to her former perfection. She also becomes prone to episodes of jealousy, particularly regarding Keita's close friendships with other girls like Chiaki Hoshinomori.

Karen’s role in the story is as a central catalyst and a primary romantic interest. Her initial approach to Keita sets the entire plot in motion, and her subsequent feelings for him make her a key participant in the series' famously tangled web of romantic misunderstandings. Her key relationships are defined by her connection to Keita, whose casual, fun-first gaming philosophy directly challenges her own competitive nature. Over time, she begins to learn from his perspective, placing more value on simply enjoying games, while he in turn develops a greater appreciation for the spirit of competition. She shares a supportive, if often bewildered, friendship with fellow club members like Eiichi Misumi and a more complex, understanding relationship with Tasuku Uehara, who becomes an early confidant of her feelings for Keita. Her friendship with Chiaki is initially strained by mutual jealousy, but they eventually work to resolve their misunderstandings.

Throughout the series, Karen Tendo undergoes significant development. She evolves from a seemingly flawless but rigid individual whose identity is tied to perfection and competition into a more rounded person who accepts her own vulnerabilities. Her romance with Keita forces her to confront her inexperience with love, her jealousy, and her difficulty handling situations that do not go according to plan. While she remains a formidable and driven person, her experiences help her relax her strict self-image and embrace a more casual and honest approach to both gaming and relationships. Among her notable abilities are her exceptional skill across a wide variety of video game genres, her top-tier academic and physical performance, and a surprisingly strong will and determination, as shown by her refusal to give up on recruiting Keita even after being rejected.