I have a related question. If I have a girlfriend that wants me to level her character, can I do it? Is it against the user agreement?
The EULA states you cannot share accounts. While it is against the rules, I don't think you will be banned by it because it is too hard to trace for too little impact in the game. The purpose of that rule is largely to prevent account sharing that results in one of the two being unable to log in after the other compromised the account completely. It's mainly there to protect the players.