Character depends on how you define a "character". If you mean a standard type player entity, then no. But if you make a weapon model that functions as a playable character, then you can easily switch characters. Map one of the buttons to a freespecial, and use weaponframe to switch characters.
Sure you could do that, if you're masochistic. Creating a daisy chain of characters duplicated as weapons just to avoid script? Knock yourself out. I'd rather watch paint dry.
That solution uses script. You simply cannot change characters (in the conventional sense) during game play without it. You can chose between stages, which is a built in (and IMO very underutilized feature), but NOT during actual game play. The engine has no such commands or allowances.
Another thing to add about weapon is if your weapon doesn't include model swap IOW just change projectile like most shooter games do, you can use script to switch that projectile based on available weapons.