Unseen

Villains who are not at all or have yet to be seen (such as William Afton, The Prototype, Blair Witch, The Man from Bambi, AM, Imu, The End, and Death) or villains who lurk in and strike from the shadows (such as Diavolo, The Phantom Animatronics, Robbers from The Incredibles, and Ernst Stavro Blofeld)

These villains tend to be either very dangerous or very mysterious. Sometimes both, given that they lurk in the shadows and can use their minions for their own dirty work. Some villains prefer remaining hidden away from others, and it often allows the story to build up to the chilling moment when they appear in the flesh (i.e. Slade, Ozai, and All For One).

It can also allow for other characters to tell and mention their supposed evil deeds without said villains making an actual appearance (i.e. Demon King, Duke of the Fallen Federation, Diamond).