Bill is a delight. He's big, round, golden and fluffy, with a beard, long rabbit ears, a purple bow tie and a grin that takes up most of his face. He's funny. He's positive. He's terrific company, and people like him straight away. That's the point.

It's a coat. Under all that gold fluff there's nothing but shadow, dark and dripping, wearing a friendly Stygian like a suit. Mimics always try very hard to look normal. Bill tries very hard to look adorable, and he is very, very good at it.

He's hungry, and he's horrible. Bill gathers souls. He's sure there's something out there, some deeper understanding of how all of this works, a kind of magic nobody else has had the nerve to reach for, and every soul brings him a little closer. What he'll do when he gets there, he hasn't said. He doesn't seem to think about that part.

He hasn't rested in a very long time. He isn't sure he can. Things have happened to him, bad things, final things, the sort nobody walks away from. He always comes back.

There was a Henry. They were friends, the best of friends, and together they built things nobody else could have built. Then Bill wrecked it, the way Bill wrecks everything.

He's been looking for Henry ever since, through every kingdom and every nightmare, for longer than anyone should look for anything. He tells himself it's to end this. That isn't why. He loves him so bitterly he can't stop, and he has been stalking him through the ages, and he will keep doing it for as long as it takes.

He still builds things. Machines, toys, contraptions, things with a lot of springs in them. Some of them are for entertaining. Some of them are for luring. Some of them are for keeping things that really should have moved on.

Sometimes Bill sees a rabbit. Golden, grinning, always just at the edge of the room. He calls him Springy, and talks to him like an old friend, and sometimes takes his advice. There's nobody there. There never has been. Springy is a piece of Bill that got loose.

Under the coat he's a Mimic: a deer-shaped body made of shadow, dripping and going see-through towards the hooves, with drooping wings fastened to the fur. A purple ghost-flame burns over his head, smaller ones run down his back, and when he wants to he can fade out entirely, which is useful for a man in his line of work.

His human form is a big, heavyset, bearded man with brown hair, very pale, and always dressed far too nicely, usually in purple.