She's not a villain, actually; her kingdom is a prosperous place, and she's kept it that way for almost a century. It's said she once knew the ancient heroes of Medieval Soldiers and even had to kill one in her time.
Although the princess is nearly a thousand years old, she still maintains her youth through her dark magic.
Dark magic doesn't symbolize evil, but since it's the only magic linked with the demons of chaos, hardly any kingdom accepts them. Thus, the Dark Princess has established her own guard, the Shadow Guard, a military entity that watches over her people from the shadows.