It is said that Sayoko committed many murders in Tokyo. Especially, she liked removing her victims' brains. Many of her victims were said to be mature women. Sayoko claimed that she lost her parents at a young age. Thus, she inherited their fortune. After selling a land which was her family's legacy, she got lots of money because of it.
Sometime before the arrival of Miina Isurugi, she got along with her cellmate and the previous owner of bracelet #8, Sara Sunagawa. She then killed Sara and removed her brain. In Chapter 46, it is hinted that she was not aware when she killed her due to Medusa Syndrome.