Nikoru Yamana is a high school student and the best friend of Runa Shirakawa. She has light-colored dyed hair, often styled in a gal fashion, and wears accessories such as a necklace and ear piercings, with pink nails. Her sharp eyes can give her an intimidating, delinquent-like appearance, which initially leads Ryuuto Kashima to fear her as a demon gal.

Despite her somewhat stern expression, Nikoru is a gentle and deeply loyal person at heart. She is protective of Runa, especially concerning her friend’s past relationships and reputation, and worries about Runa getting hurt. She watches over her like a parent, offering advice to both Runa and Ryuuto to ensure their relationship goes well. Her motivation stems from a genuine desire for her best friend’s happiness.

Nikoru aspires to become a nail artist and hopes to attend a nail technician school after graduating. Because she was raised by a single mother, she works part-time jobs after school to avoid placing a financial burden on her parent.

In the story, Nikoru functions as a key supporting character and a close confidante of Runa. She is initially skeptical of Ryuuto, seeing him as an ordinary and unremarkable boy, but she gradually comes to trust his sincerity and becomes one of the strongest supporters of their relationship. Within her circle of friends, she is close to fellow gyaru Akari Tanikita and Ichigo, and she develops a dynamic with Yusuke Ijichi, a friend of Ryuuto, marked by an opposites-attract interaction.

Nikoru’s development centers on her shifting attitude toward Ryuuto—moving from cautious disapproval to protective acceptance—and her continued role as a loyal guardian of Runa’s well-being. She does not possess any special combat or supernatural abilities; her notable traits are her sharp observational skills, firm sense of loyalty, and a pure, caring nature hidden beneath a tough exterior.