Persona is medieval recreation slang for who one is with regard to the medieval environment. It does see some use at Renaissance Faires as well very common usage within the SCA

In the Rennie sense, it may be used to refer both to what role each bit of paid help has as well as who patrons who come in garb are.

Within the Society for Creative Anachronism, things are more complex. To an extent, everyone is expected to have a persona, usually the following as a minimum:

Within the SCA, much of what one does will tie in with your persona. This includes the clothing you wear, the utensils you use to eat, and even how you act in response to royalty.

There are several suggestions about where to start with a persona. Some say you should first look for a form of costume you can stand wearing quite a bit, and then find out where and when it is from. Others suggest you start looking at your ancestors. Others yet suggest you find a culture you like, and pick their time.

Once this is done, you can sketch out the persona's past, the deeds that they have done, their social class, and other things to your heart's content. Warning: Some people feel that the persona allows for too much roleplaying, allowing one to claim having done things that the real person could not -- for instance, great skill in battle. I would say that it wouldn't hurt to be modest.

A few web sites that get into it a little further:
On the Renaissance Fair side: http://www.atthefaire.com/persona.html
From the SCA perspective: http://www.ealdormere.sca.org/persona.html, http://www.interaktv.com/HISTORY/SCApersona.html