There is a game system that actually works very well for temporary characters: Ars Magica. Without going into too much detail, each player creates 3 characters: a magus, a companion and a grog (fighter/shield-bearer sort). Grogs are a GROUP resource so anyone can play any one of them. Each session, a player is playing either their mage, their companion or one of the grogs. So the grogs are part of the campaign, with a suitable position and their own story arc, but are not so essential that they NEED to be there for any particular session.