A character backstory exists to give the player a base from where to start building him up.
It's perfectly reasonable to give them a long backstory. Because it's all in the character's past, it makes sense for it to be set in stone. Additionally, a rich backstory will provides more RP hooks to other fellow players.
It's perfectly reasonable to give them a long backstory. Because it's all in the character's past, it makes sense for it to be set in stone. Additionally, a rich backstory will provides more RP hooks to other fellow players.