Story

When she was born, Mei had the stigma of the Herrscher of Thunder which she inherited from her mother. Mei was unable to control the power of her stigma so Cocolia who wanted Mei alive for her stigma agreed to implant the Herrscher of Thunder core into Mei to absorb the stigma energy and prevent loss of control. Upon the core being implanted the soul within it was overridden by the soul within her stigma, a Honkai eruption would later happen in Nagazora and the soul within the core would control Mei to become the Herrscher of Thunder, but Mei would take control again ect. Once Mei was separated from the core by Herrscher of the Void the stigma soul who had stopped interacting with Mei kept doing that and observing from the outside. Once the Mei and the core were reunited in Nagazora the soul in the core agreed to be overridden by Mei so she would gain the full power of the Herrscher of Thunder, the stigma soul was still there watching on though. Due to passive observation the stigma soul became very different to the Herrscher soul and failed to understand Mei in many ways yet stayed an observer until her existence was found by Mei when she was trapped in her stigma by Gray Serpent as he tried to swap the 2 souls, but the stigma soul kept her role and let Mei keep her's.