Rei and Sei's mother, Shoko, passed away when they were both very young. Initially, Rei doesn't seem to have a strong emotional connection to her, stating that he and Sei only really relied on each other and always have.
As the series progresses, it is revealed that Shoko suffered from severe schizophrenia. She often spoke strange things to Rei and Sei, threatened anyone who hurt them in any way, and even killed a dog in front of them once because she believed it was scaring them. Covered in the dog's blood, she reached out for an embrace from Rei and Sei, but they were terrified of her.
One night, Rei and Sei woke up and went to her room to find her hanging from a noose, dead. Rei has tried to suppress the true events surrounding her death, generally believing that she died from an illness. His memories are mainly of visiting her at the hospital, although it wasn't a regular hospital but a psychiatric one.
