Riven:
I said 'appear' both for a bit of a fudge (although that doesn't exactly work with the idea of a nemesis really) and because there was the natural contradiction of an unenhanced individual having become a locus for the power of beings who had previously appeared incapable of making such a connection. I didn't have any strong ideas but thought there might have been some kind of space there for explaining things away as actually not being The Dark and having the character discover in time that their approaching end was something other than what they'd been preparing for.
Pretty much my thinking. There are some ways to play it, but... tricky.
Riven:
The Reformed (snip)
The Nautilian went into hiding in the late 70s, towards the end of the War and a few years before the creation of New Atlantis. I don't really need an exact timeline, just trying to get the sense of how big a deal this person would've been to the rest of the world. Just another minor upstart who was active for a few years a while back is different from someone who terrorised the world from the War's end to this day. That would tend to affect how others see the character.
What sort of abilities are you thinking of for your character?
Re: killing (putting this here since it relates to this character), yeah, that's also tricky. It makes the most sense to allow that for Reformed than anyone else. After all, they are defined by having done very bad things on purpose in the past, but trying to be different now. (Compared to a Nova who could very well have killed people - but not on purpose.) On the other hand, it seems likely to seriously affect the tone of the game and everyone else's experience. After all, it would pretty much have to come up at some point.
On reflection, though, I would allow it - the requirement from the core book is that the character is not someone who "solves problems by killing"
now. Also, considering the setting, sympathetic characters who have killed in the past are probably more of a given than they would be in a generic Masks game. Such themes might come up eventually anyway. So yes, you can use this tool if you think it works best for your concept, just be careful with it and be prepared for consequences. The examples you mentioned seem like a valid way of handling it (more so Cassandra since she had more extenuating circumstances and quit afterwards).
Riven:
The Transformed (snip)
Got it. By Cyborg scenario you mean it was used to save the character's life when no other options were available? I could see something like that - the technology is far ahead of Earth's, and does include tools for cybernetic conversion (that may or may not have been properly calibrated for humans, at that). Study of it is heavily restricted by law pretty much everywhere - only the governments and groups with special permissions from governments are supposed to do it, and then in maximally safe conditions. Of course, some do it on the sly anyway...
It could also just be an accident from illicit research, really. Something might have activated on its own and no one could stop it from converting the character due to insufficient precautions.