Karen Maijima is an 18-year-old character who appears as a potential romantic interest within her narrative. She is formally enrolled as a twelfth-grade student, though her career as a popular singing idol drastically limits her presence at school. Physically, she is described as having long, wavy blonde hair without bangs, amber eyes, pale skin, and a slim figure. She stands 168 centimeters tall.

Her personality blends cheerfulness, friendliness, and kindness with a strong sense of professionalism and work ethic, referring to herself with the polite pronoun watashi. Despite the demands of her entertainment career, she is shown to have a will of her own; for instance, she is capable of challenging her manager's decisions, such as raising an objection to an unexpected swimsuit photoshoot. Her relative absence from school has left her with few close friends among her classmates, making her something of an isolated figure despite her public fame.

As an idol, her professional life is a central element of her role, with a microphone being a key item associated with her singing profession. Her career is managed by a character named Hikari, who sometimes creates obstacles in her personal life. In the specific context of the story "Dōkyūsei 2 Special: Sotsugyōsei," Karen appears in a supporting role. Within this narrative, she maintains a close friendship with other characters, notably Narusawa and Yui.

Karen comes from a wealthy family and is one of several potential love interests the protagonist can encounter. Her wardrobe consists of various items including a beret, bikini, bow tie, coat, miniskirt, school uniform, thigh-high stockings, and a turtleneck shirt.