Millie The Doll is an entity that appears in The Locker Rooms. The entity is a mix between a small child, a doll, and some kind of spider, making wood-like noises when quickly moving. The entity's speed is really high, being able to cover large distances in a few seconds. Although it's fast, the main attack technique utilized by Millie is climbing up a few floors above you and jumping onto wanderers below. Once the entity is on the same floor as you, it may already be too late. To survive an encounter with Millie, due to the entity's poor eyesight, you can hide in any of the lockers or vents. If you have one, you could also splash an invisibility potion on yourself.
It is better to keep a safe distance from Millie and listen for any form of giggles or laughter that seem to be coming from a small child.