This is why my D&D kit contains a folder full of backup characters, one at each level up to 20. They're all core races and classes, at standard point buy, so they can be dropped into nearly any campaign. I also carry a few dozen more on a USB jumpdrive, so I can print them out if there's a computer nearby.
But I guess not everyone's as big a nerd as me.
If there are no backup characters available, the newcomer can read over my shoulder and play a copy of my PC. Maybe he's some random passer-by who happens to be of my race and class, or perhaps he really is my identical twin (or a duplicate created by a teleportation accident, or a copy of myself from an alternate timeline, or whatever).
Somebody insisting on actual control over a favorite long-running character would make me really suspicious. The world contains an infinite supply of characters, you can easily have one of your own, so the only reason to insist on
this exact one is if you've got some bizarre personal agenda-- like a a D&D version of
Indecent Proposal. I don't care if you get off on power, but I'm not letting you do it at my expense, thanks.