Shigure is Rei's bodyguard and Okumiya's romantic interest. Looks up to his older brother to a fault. In volume 4 he becomes Raoul's bodyguard, for the moment....
It is mentioned that Shigure looks up to Okumiya as he does his brother. Though there is a hint of something more.
Akira calls him Mr. Shigure, while Yuji and Okumiya call him Shigure-kun.