Blue eyes / base Shigure has a heartlock earring on her left ear lobe while Kai Ni Shigure doesn't. I guess Kai Ni Shigure is the combat Shigure while the base Shigure (while still part of the main fleet) is the Admiral's favorite.
The Admiral's a good person.Everyone trusts him.He's serious, kind, and doesn't have favorites...EndJust kidding....to you, anyway.