Who has Celty's head in Durarara?

See, I've been into this show for like 3 days and I'm on episode 5 but have looked up a lot of things on Wikipedia and images, so I know Celty's background story and what her head looks like. I am REALLY confused though. Who has her head? The lady with the hat? And if someone has her head on, how did it get on? And also, can Celty talk? Or can she talk, but also uses the PDA at the same time for some odd reason...? Please, I'm so confused!