Boo

Boo is an enemy in the Super Mario series of games. In Super Mario 64, he appears in the Castle Courtyard and Big Boo Haunt. They can also appear in the Fourth Floor wandering the endless halls. In very personalized copies a toad from the Internal Garden can give you a quest to kill them. That, rewards you with a star

Theories
A possible theory related to the Boos is that the courtyard ones are spirits of dead toads. Similarly, it could be possible that the fountain in the courtyard is a memorial for someone or some group, and the boos are their spirits. it is also theorized that they could be the souls of bowsers minions that still want to help him