Karen is Tousei Kaneshiro's mother, an excommunicated priestess and later an escort within the Ryūga-verse. She already died before the events of The One Who Shines in the Darkness as Tousei discovered her mother's connection to the Makai Order and used her magic to summon a horror and devoured her. Tousei would mis-use her mother's makai tomes to further his agenda of revenge.
Personality & Character
Much of Karen's personality remained undefined. It was never explained how did Karen ended up excommunicated by the Makai Order, but it made her become jaded, narcissistic, and disillusioned with her life. In order to survive in the regular human world, Karen became an escort. She used her beauty as a means to make money and manipulated men to help her financially survive. Through her journey, she gave birth to Tousei, but Kensui Kaneshiro already have a family and disavowed Tousei's existence. Karen continued working as an escort, hiding away her priestess past until Tousei secretly discovered it. She had a poor sense of discretion with Tousei and she escorted right within her own home, where Tousei is able to witness her service her clients and it sickened him. Because she often drinks with men, Karen often appears drunk and belligerent in front of her son. She cares about Tousei, but only reinforced the idea of him becoming a great man without helping him. Her own past would ultimately end her as Tousei had enough of Karen's ways and used her own magical items to kill her.
Nothing was explained about Karen, only that she was once a priestess and was later excommunicated from the Makai Order. Tousei said she was exiled from the Order, suggestively she violated a major rule and removed from the Makai Order. Although exiled, she still kept a chest of her belongings of magics and tomes during her priestess days (don't know why would the order would just let her keep these things as they're highly sensitive materials). It's unclear why did she become an escort, but suggestively she had no other life skills other than possessing being a beauty and used it to financially survive.
Meeting Kensui & Raising Tousei
Sometime during Karen's escort past, she met Kensui Kaneshiro and became pregnant with her first child. Unfortunately, Kensui is already married with wife and kids. He couldn't afford the scandal of the public knowing he bastardized an illegitimate son he and he disavowed him. It's not clear how did that affect Karen, but she continued on her escort path while raising Tousei in between.
Tousei resented his life as he couldn't stand his mother's escort job. He sees her as a whore and resents his father for denying his existence. Filled with resentment and anger, he channeled all of it into revenge when he discovered Karen's priestess past. Secretly studying her makai tomes, Tousei learned a great deal about the Makai Order and gained knowledge of their culture. To test his understanding of makai magic, Tousei used magical tags to call forth Horrors to devour everyone inside his house. Karen along with the men she was servicing were all devoured, except for Tousei. He created a barrier to protect himself and that began his dark path in vengeance against the Kaneshiro family. Karen would be remembered as a filthy whore in her own son's eyes and died without understanding why.