Karen Ayugase is a supporting character who is a student at Royal Sakuraka High School and the best friend and classmate of Akane Sakurada. She is also a childhood friend of Akane, having known her since they were young.

In terms of appearance, Karen is a teenage girl of average build and height. She has short, straight black hair that falls to about her neck, which she often styles with a small ponytail tied on the left side. Her bangs are mostly parted to the right, and she has blue eyes and a fair complexion. She typically wears the school uniform, which includes a light-yellow sweater over a white shirt with a red tie, paired with a red skirt and black knee-length stockings.

Karen is characterized as a kind and caring individual, particularly attentive and protective of her friend Akane. She is known to be somewhat serious at times and can become embarrassed or shy when receiving compliments. Her relationship with Akane is the central aspect of her character. The two share a deep bond of trust, and Karen is often shown worrying about Akane and encouraging her when she feels down. This loyalty is rooted in a childhood incident where Akane defended Karen from burglars, an event that left a strong impression on her.

Her role in the story is primarily as a close friend and confidante to Akane Sakurada, one of the royal siblings with supernatural abilities. The narrative often suggests that Karen might harbor romantic feelings for Akane. For instance, she experiences a nosebleed when inadvertently seeing Akane's underwear, a common trope indicating attraction, and she becomes visibly upset when Akane jokes about wanting to marry her. Beyond this, Karen is also shown to be friendly with another character, An Shirogane, as they are seen together encouraging Akane.

Karen does not possess any notable supernatural abilities of her own. Her significance in the story comes from her emotional support of Akane and her role as a grounding presence in her life.