Probably she's heard of Romana (perhaps she's a friend of that player), and knows that she's left, so she sees an available niche, and designs a character that fills much the same function. Which brings us to Nyssa of Traken, who does, of course, become a regular PC from Logopolis onwards.
Nyssa, like Romana, comes from a technologically advanced culture. Quite how advanced is hard to say, because Doctor Who doesn't, if we're honest, have a terribly consistent view of what RPGs call "tech levels". With a few exceptions here and there, there doesn't appear to be functionally much difference between any of the futuristic societies we see. They may emphasise different bits, sure, but so long as they've got starships, ray guns, and any specific gadgets needed to drive the plot (the miniscope from Carnival of Monsters, say) they mostly look pretty much the same. Even the Time Lords don't seem that different, apart from the fact they have time machines.
