The long answer is we are not allowed to have more then one character. This is to force people to interact and not allow one player to form his own faction/group without anyone else. Allowing more then one character could also create balancing issues.
I would highly suggest you take some times to read the rules:
1/ Multi Accounts
Every member of the combine may only have ONE character. Registering more than one character, logging into someone else's account, or giving your character to someone else in order to create a new one is PROHIBITED. Anyone breaking this rule will be banned from the Combine.