Rose grew up in Caldera until she didn't, because things went south with her adopted dad, when their military unit didn't protect a Calderan VIP. Thus, they became a mixture of merchants and secret mercenaries/assassins/et cetera. So she could have known Ryuko, because of orphans in Caldera!

With things to discover later. They might be friends? Rose is pretty go get 'em and intuitive, if not quite as shouty as Ryuko is. (But who is?)