Royal guard is a group of monsters that are dedicated to defending the Underground, protecting Asgore, and searching for fallen humans.
Royal guard is supposed to protect everyone, but most of the time they just attack random monsters. Most members are sadists who don't hesitate to kill anybody anytime. The captain of Royal guard, Undyne, is most powerful and sadistic of them, but Papyrus is trying to reach her place. Every monster in Underground is afraid of the royal guards, yet they appreciate their work and that they are doing the best they can.