She was born with a twin brother, Rui Yonagi. She often pays attention to her older sister because of her strange behavior when acting or watching a film. She said that Yonagi must become an actress because if not, she is just a scary person who turns into a different person a whole day because of her method acting.
Rei has light-colored close to shoulder length hair with a single protruding strand of hair in the front and center, she also has light-colored eyes. In her first appearance, she wore a semi-dark dress.
Rei has a playful and innocent personality. He looks up to his sister and hopes that one day she accomplishes her goal of becoming an actor.
Rui and her twin brother Rei greet their big sister Kei home as the housework rangers. Later the two enjoy a bath together and learn that not only did Kei not get picked for the audition but also was fired from her job. Kei then tells her siblings that she's resolved herself to get a real job and quit trying to become an actor. At bedtime, the twins are asleep, while Kei reminisces when their mother was still alive. Rui is shown to be still awake and hopes that her sister doesn't give up on her dream of becoming an actress.
The next day, Kei acts in front of the twins not to prepare herself for another audition, but for getting a job. An emotional Rui tries to get her older sister to pursue acting again, until the arrival of Akira Hoshi. Rei instantly recognizes Akira as his character Ultra Masked Man. The interreaction between Rei and Akira causes a scene, making them all get into Akira's car and leave the scene. On their way to the place of the audition, they learn of the circumstances of why their sister was chosen to be a fill-in for the last round of an audition. The twins watch their sister put on a magnificent performance, but it's later learned that she didn't win the part. At their home, the twins try and console their sister, but then the director Sumiji arrives and gives her an offer.