[quote=@LeeRoy]
[@ImportantNobody]
What if they're...

What if the characters aren't real? 

Like, Smash Bros? 

But not dolls.

They're weaker copies of the original character, thrown into a wholly new world.

So they can be the character with reduced versions of their own powers, born into the world where the roleplay takes place?
[/quote]

Heroes from the multiverse have their essence collected by some cosmic entity and placed into empty vessels that take on their form and some of their power. This will also explain how they come back to life as the essence gains a new empty vessel to use. The taken essence is its own entity from the hero it was taken from even if originally meant to be an exact copy. They can learn and remember even if they die and transfer to another vessel. The essences are sort of like mercenaries because they can be summoned to aid in conflict across the multiverse. 

The ability to be summoned is built into their vessel so they must obey the summon, although perhaps later on can develop a method to free themselves from such obligation.

This is similar to an anime series but has plenty of original ideas. In the anime its past heroes categorized into different classes such as archer and berserker and they fight over a McGuffin. For example, you can summon the anime's interpretation of King Arthur.