Rei and Sei's mother who died while they were both a young age. At the beginning of the series, Rei doesn't really seem to have much of an attachment to her, saying that him and Sei only really relied on each other and always have.

Spoiler

Later on in the series it was revealed that Shoko was actually very schizophrenic. She often said strange things to Rei and Sei, seriously threatened people who remotely hurt them in any way, and even killed a dog in front of them one time because to her it seemed to be scaring them. Covered in the dog's blood, she extended her arms out for an embrace from Rei and Sei, but they were extremely scared of her. One night Rei and Sei woke up and went in her room to find her hanging from a noose, dead. Rei has tried to block out what truly happened to her and generally thinks that she died from an illness because all he really remembers is always going to the hospital to see her (which really wasn't a regular hospital, but a psychiatric one).