Diplomat-verse Luna is Celestia’s personal bodyguard and enforcer. She answers only directly to the empress, has the authority to recruit any non-civilian units to her cause and can override any commands not given by the empress herself. Usually she is just a silent figure in the shadows, watching and listening, but when demanded, she’ll pursue criminals by whatever means necessary and bring them to justice. What many people don’t know is that she’s also Celestia’s sister and could replace her in case of an emergency.  
A few years ago she was temporarily banished from the kingdom for allegedly partaking in a plot to murder the princess, but has since been cleared of the charges and returned to her duties.  
A sort of religious awe seems to follow her around because of her efficiency and closeness to Celestia. “The moon is always in the shadow of the sun”, people say, and utter a quick prayer that she’ll protect them as she protects the monarch.