Who are you in the SCA? And what is a persona?
In the SCA we take on an alternate persona, that of someone who might have lived prior to 1600AD. We take on a different name, we sometimes dress in suitable garb, and we sometimes study the period we are supposed to have lived in. Nearly everybody chooses a persona (0r SCA) name, and for some that is all they choose to do. Others may go into devising a whole history for their new persona and go to great lengths to be authentic to that persona. What you do is up to you. It depends on how much effort you would enjoy putting into it. You can be an Anglo Saxon woman, a Viking raider, a Scottish mercenary or an Italian Renaissance man - the choice is up to you.
There are some restrictions on what sort of name you can choose. It cannot be intrinsically offensive (like Basil Babystabber) or contain obscene terms. It cannot be the real name of an actual historical figure (like Richard the Lionheart or Anne Boleyn). Neither can it be a famous figure from modern fantasy (like Gandalf, Luke Skywalker, or Belgarion). This still leaves you with many options. You should try to choose a first name that you like and that you will answer to (because people will be calling you by it). It should also be something that can be documented as being period. Your surname can take one of three forms. It can be a place name (like Eleanor of Cathenes or Glynyhvar of Riverhaven). It can be a descriptive name (like Peter the Uncertain or Catherine the Friendly) or it can be a proper surname (like Douglas Fitzwilliam or Blaine ap Mayo). It can be in any cultural style that existed in period.
Things you can't do
You can't take on he persona of a person of title or rank and you cannot use a name that might suggest that. You can't call yourself King, Duke or Early as they are SCA titles which must be earned. You can't take on the persona of a knight or a lord or a lady as these are honours that you have not yet been given. It is considered that everyone starts out in the SCA as untitled lesser nobility and from there all titles and honours must be earned.
Acting out a persona
It can be fun to act out your persona. A conservative accountant can become a wild Scottish warrior or a bank teller may become a pampered Elizabethan courtier. This is the fun part of Persona.
Persona can cause problems if taken too far. We consider everyone to be noble - we treat no one as a villain or peasant. If the 12th century Crusader and the Islamic tribesman take their roles too seriously conflict may result. This is why, in the SCA we are all at peace, no matter if our chosen cultures are at war.
We also try not to disparage religious beliefs, even in jest. The SCA has Christians, Muslims, Hindus, Pagans and every other religion under the sun, and it is not right to offend peoples deeply held beliefs.
The main yardstick of persona is if it offends, don't do it.
Why bother with Heraldry?
The use of heraldry in the SCA adds to the medieval atmosphere. The brightly coloured banners blowing gently in the breeze. The unique designs on fighter's shields, the use of heraldic devices in garb all contribute to the look of the SCA. Further information on Heraldry can be found here on the Heraldry Page