Karen Moriya is a high school student who has been a close friend of Chinatsu Kano since their kindergarten days. Among her peers, she is one of the few people who are aware that Chinatsu lives under the same roof as her classmate and badminton club member, Taiki Inomata. Unlike most of the other characters who are solely focused on their school club activities, Karen balances her academic life with a career as an entertainer in show business.

In terms of personality, Karen is portrayed as a romantic individual who is equally dedicated to her own personal goals and dreams. She composes herself in a manner that comes across as mature and more aligned with the adult world compared to the other young characters, often displaying a frank and practical outlook on complicated situations. Her exasperated but affectionate way of pointing out that everyone is overthinking things highlights her role as someone who cuts through confusion and encourages straightforwardness in emotional matters. Her supportive and loyal nature is particularly evident in her friendship with Chinatsu, whom she cares for deeply.

A key driver for Karen is her desire to support the people she cares about while also cherishing her own path. Her relationship with her boyfriend, Kengo Haryu, serves as a central part of her story. Together, they represent a notably healthy and honest portrayal of young love. Karen’s role in the narrative frequently involves acting as a confidante and a mirror for the main protagonist, Taiki. Karen and Haryu demonstrate that love requires active choices, communication, and a refusal to become paralyzed by overthinking, which is a direct contrast to Taiki's often hesitant and internally conflicted approach to his own feelings for Chinatsu. As a close friend of Chinatsu, Karen is also a trusted person with whom Chinatsu can have honest conversations about her evolving feelings for Taiki as more than just a friend and housemate.

Karen's key relationships are clearly defined. Her bond with Chinatsu is her longest-standing friendship, acting as a pillar of support for both girls. Her romantic relationship with Kengo Haryu is well-known among their friends and has been building since they first met in the fifth grade. Their relationship is not recent or secret; it is established and serves as a model for what a committed partnership can look like during the high school years. Karen also has a younger sister named Ayame Moriya, who is very close in age to her.

The development of Karen's character is primarily explored through the lens of her relationship with Haryu. Unlike the central pair, whose story is defined by pining and uncertainty, Karen and Haryu are depicted as having already navigated the initial hurdles of confession and are now in a phase of actively building a life that accommodates both their individual ambitions and their shared affection. Flashbacks reveal key moments in their past, such as how Haryu won Karen's heart by bringing her a journal to cheer her up after a bad day, illustrating that their bond was forged through considerate actions rather than grand, movie-like gestures. This backstory reinforces her character as someone who values practical effort and genuine connection.

A notable ability of Karen Moriya is her capacity to function as an emotional anchor and a source of clarity for those around her. Her perceptiveness and maturity allow her to understand the romantic dynamics between Taiki and Chinatsu, and she actively roots for them to find their way. She is not merely a passive observer; her own secure relationship provides a functioning example that challenges the more anxious and idealized notions of love held by the main characters. Her frank communication style and refusal to treat romance as an impossible puzzle make her an effective, albeit sometimes blunt, advisor and friend.