Straight Laced is part of Luna's security detail and is excellent at magically and physically restraining individuals who might attempt to threaten the Princess or other members of the Night Court.
Upon establishing a proper Night Court in Canterlot Luna quickly realized that many ponies might have forgiven her for becoming Nightmare Moon, they still imagined that her court was like some goth masquerade. Initially she tried to convince ponies otherwise, however she found that many of the ponies who were suitable for her court fit that description. Heck she was happy that ponies were actually applying for posts. So instead resisting the whole goth image Luna decided to own it instead and has embraced it along with her staff.
At least she can recognize her staff from her sisters.
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