Yui is Rei's older sister. She moved to Tokyo after joining the workforce. Rei's Girlfriend sees her in front of Rei's house speaking with his mother and grieving over his death and cherishing the necklace he gave her before his death. Rei's girlfriend first thinks Rei had an affair with her (not knowing she's his sister) so when she gets back to her house she is furious with Rei and says she has no choice but to kill Yui, to which Rei says she's his sister and the necklace was something he gave her in elementary school.
My Yandere Girlfriend Won't Let Me Sleep
Rei's girlfriend has summoned him back to the living.
