During the final episode of the first season of Rozen Maiden (the original anime series), Jun repairs Shinku's arm and someone (i think it was Suigintou) act surprised by this calling Jun a Maestro.
What exactly is a Maestro and is it related to Jun being Shinku's Medium (ie, so any Medium is a Maestro to their Rozen Maiden)
Being a Maestro is separate from being a doll's medium.
A maestro is someone who has special abilities in creating and repairing dolls. (This becomes particularly apparent in the latest Rozen Maiden manga series.)
If you watch enough of the anime or read enough of the manga, you'll notice only a couple of people are mentioned as being a maestro (Rozen, Jun), and they're the only ones who are ever seen creating or repairing a doll (as opposed to just the clothes).
If you want a full explanation with all the evidence laid out, check out the AnimeSuki Rozen Maiden Q&A
