"Kagero, reporting for duty Milord!"

“As Lord Ryoma’s retainer, it is my duty to follow his every order. He wishes to take you back to Hoshido, and so it must be done.”

Kagero from Fire Emblem Fates.

Wikia Time
Kagero is a kunoichi from a noble family who has served the Hoshidan royalty for generations. Originally one of Queen Mikoto's personal retainers, Kagero later became Ryoma's retainer at her liege's behest. According to her supports with the Avatar, her position was originally meant to be assumed by her older brother. However, due to him suffering from a disease that prevented him from executing the duties expected of a ninja, she thus made the decision to step forward and serve the Hoshidan royal family in his stead.

In the support conversations that Kagero shares with her fellow retainer Saizo, it is revealed that the two of them were in a romantic relationship at some point prior to the events of Fates. Due to their inability to reconcile their differences, they eventually elected to end the relationship and focus instead on their duties.