Shuko is Misaki's mother and Shoko's elder sister. She is 31 years old. In the anime, it is explained that Shuko left a 5-year-old Misaki to be taken care of her grandparents under the guise of work, when in reality she was going to Tokyo in order to find a way to cure the neurological disease she had been inflicted with. This disease inhibits nerve signals from reaching to or from her legs. Ichiro Mihara was a doctor attempting to work with her to find a cure when the two started Angelic Layer as a way to do research and raise funds when their original research funding was cut off. Shuko, or "Shu" as she is called when playing Angelic Layer, is the undisputed champion with her angel Athena, having won each of the past three national championships. Over the past seven years, she has avoided meeting Misaki, afraid of what Misaki would think of her. In the manga, however, Shuko has no such disease and is merely extremely shy about socializing with those she loves. She also works for the company behind Angelic Layer.

(Source: Wikipedia)