I'll ask here since it's coming up a bit now for [@Master Bruce] and [@Lord Wraith]. Where does the GM stand on establishing 'title characters' to fulfil other people's stories?

Per Examplé (Read it in a French accent, it's better), if I was playing The [I]Flash[/i] and there was no [i]Martian Manhunter[/i] player, yet I wanted to use that character to tell a story, how much of [I]Martian Manhunters[/i] backstory could I elaborate on?

I guess I'm just worried that at some point someones going to establish something that will put someone else off applying.

[b]Edit:[/b]

Ew I posted. As always feedback appreciated.