Your reasoning has so far been "because I don't think it's necessary", that's a pretty lazy reason, and I think it's because there really is no good reason for you to not put either in. I can get behind an RP that really doesn't require either and plays like a rogue-like, or is a short RP for the sake of doing certain or particular types of roleplay that doesn't need a rich story or world.

But this is neither of those. This is an RP that has multiple players and characters, in a fairly decided setting, that has it's own lore and world to draw from. A backstory and personality is very important, especially the backstory.

Just because you don't think it's necessary doesn't mean it actually isn't. I'll let the GM decide on this, but I've made my case.

If it's in the vein to keep characters a mystery, that's even worse. There should still be some way your character acts, like an enigmatic warrior can still show to be honorable, yet extreme, but still hide their true colors, and backstory? Your character has got to have made a name for themselves somehow, or been known some way. If there are secrets to their backstory, simply be vague or omit those parts, and send them to the GM.

Basically, even if you wanted to omit stuff or keep yourself a mystery, you can STILL do that. Mysterious characters in literature aren't mysterious because we know nothing about them, it's because we know a little about them, and what little we know, makes us want to know more.