Yui Yuigahama attends Sōbu High School and joins the Service Club as its third member after being its first client. Her childhood habit of prioritizing social harmony, such as accepting undesirable roles like "the dog" to avoid conflict despite wanting to be "mother," continues into high school. She maintains superficial friendships while fearing peer rejection. Her life changes significantly when Hachiman Hikigaya saves her dog, Sablé, from a car accident on their first day of high school, resulting in his hospitalization. Yui visits his home with thank-you gifts, but Komachi consumes them without conveying the message, delaying her connection with Hachiman until their second year.

Initially perceived as a cheerful, popular student with strong social skills, Yui excels at reading social dynamics and adjusting her behavior to maintain group cohesion, acting as the "glue" within Hayato Hayama's clique. This masks an internal conflict between her desire for genuine connection and her fear of disrupting social equilibrium. She often suppresses her true opinions to avoid disagreement, describing herself as someone who "always wants to fit in" and feels anxious when disagreeing with friends. Her initial attraction to Hachiman stems from seeing him as an embodiment of her idealized self—someone willing to act selflessly and speak bluntly, qualities she admires but feels she lacks. This admiration evolves into romantic feelings.

Interactions within the Service Club drive her development. Hachiman's rejection of "nice girls" and pursuit of "something genuine" challenges her persona, forcing her to confront the disparity between her outward kindness and inner desires. Key moments include her tearful reaction when Hachiman attributes her kindness to guilt over the dog incident, her failed attempt to monopolize Yukino's friendship while distancing her from Hachiman during the three-way date, and her gradual realization that Yukino and Hachiman share mutual feelings. At the cultural festival, she demonstrates growing assertiveness by volunteering for the student council presidency, partly motivated by Yukino's involvement.

The climax occurs during prom planning. After Yukino cuts ties with Hachiman, Yui grapples with conflicting impulses: her selfish desire to pursue Hachiman versus her selfless wish for Yukino's happiness. Meeting with Yukino, who confesses her feelings for Hachiman, leads Yui to acknowledge her own love while resolving to remain friends with Yukino. In a pivotal conversation with Hachiman, she encourages him to prioritize understanding others over seeking a perfect "genuine" connection, influencing his decision to reconcile with Yukino. Ultimately, she confesses her feelings to both, accepting Hachiman's choice of Yukino while vowing to maintain their three-way friendship. Her final resolution involves accepting both her "kind" persona and underlying selfishness as valid aspects of her identity.

Throughout the series, Yui demonstrates emotional intelligence, persistence in maintaining bonds, and capacity for growth. She evolves from hiding behind social conformity to expressing difficult truths, exemplified by her confession and acceptance of rejection. Her journey concludes with a commitment to honesty in her relationships.