Shuichi is an acquaintance of Shirogane's, and owns a bar named 'Bar Still'. He is commonly called "Master". Even though he is blind, his Reiken allows him to sense spirits and Kokuchi. He also has an ability that allows him to cure others, albeit a little painfully.
In the manga, he is revealed to be the other King of Rei, Shisui, who went into hiding.(http://en.wikipedia.org/wiki/Monochrome_Factor)