Another determinant would be just how well-known this last-mage's existence is or has become. If she just recently came into her talent and only the rest of the party (if even they!) know about it, that's far different - and far easier to deal with in play - than if it's known far and wide that Jocasta is the last known mage in the world.
My point is that it doesn't matter what I write adventures about. The adventures could ignore the last-mage piece entirely and yet most of the time it'd still be top-of-mind for all involved: "We have to protect Jocasta, she's the last mage." "Does anyone in this town know what Jocasta is, i.e. how much care do we need to take?" , and so forth.
You're making a lot of assumptions. Why would the PCs automatically think to protect her? Why would they even care about her? Depending on what the world thinks about magic--decided by the GM and other players during world creation--their viewpoints could be anything to "she must be protected" to "she must be killed" to "who cares?"
In the Discworld novels, for instance, wizards are generally considered to be fairly useless--they perform amazing deeds, sure, but most of those deeds are utterly useless from the point of view of the populace and they can be safely ignored unless they happen to tear open a hole in reality, in which case they better fix it before it spooks the horses. Discworld wizards the equivalent of theoretical physicists--what they do has practically no bearing on anyone's day-to-day lives, no matter how awesome it might be. Even most of the wizards themselves tend to put more stock in their staves, which they can use to bonk their enemies on the head, then in the spells they cast.
In a more prosaic, RPG sense of the world, what if the reason there are no more mages is because they all killed each other off in the wizard wars? People won't care if there's only one left, as long as she doesn't start killing non-casters. If casters had been snobbish to non-casters, then good riddance to bad rubbish. If casters had been known to dabble in dark forces and unleash horrible things upon the innocent populace, then the faster Jocasta dies, the better.
The GM is also going to have to give some thought as to how - or if - Jocasta's abilities can increase or expand in step with the rest of the PCs. The improvements among the other PC can be easily explained in the fiction through training, practice, and so on; but any new abilities or spells for Jocasta are probably going to have to come to her spontaneously...which, depending on system, might force the GM into using or devising mechanics she otherwise might have not.
That's really easy: she now has new spells and magic points. I'm pretty sure 99% of GMs will do it that way, and the idea of creating an entire subsystem for an NPC mage seems laughably useless and outdated. It's not "more fair." It's just more work.
In this case it wasn't just the one player. That, and I'm fully in the "do what the character would do" camp both as GM and player; and when what two (or more) characters would do is diametrically opposed then I've no problem with that being sorted in-character to whatever extent is required.
Firstly, "do what the characters would do" just means you need to make characters that aren't jerks to the other players. (and by
jerk, I mean something a lot coarser)
Secondly, in our group's D&D game, I've managed to play a chaotic neutral tiefling rogue for quite some time without being a jerk to the other players. Why? Because chaotic neutral does not mean that I force my decisions on other players.
Thirdly, there is no "what my character would do," because you, the player, are making that decision. Your character is just a bunch of numbers on a page.
You control their opinions and actions.
It's no coincidence that so many RPG horror stories start with players who are jerks but try to excuse their behavior with "it's what my character would do."
Er...you might have missed that the PC that was forced to continue play was mine; which means it did affect me.
No, I saw it. But the thing you balked at having your character "guard" another character by forcing them to remain "safe" at home, because that meant your character wouldn't be able to do other, presumably more fun, things.
And the bodyguard piece comes back to doing what the charcacter would do. If I'm playing a warrior-type who's at loose ends and looking for a place in the world and you present me with this task/duty then sure, I'm your bodyguard as long as you need one. But if I'm playing a free-thinking scout type with her own goals and ambitions and who doesn't want to be tied down being someone's protector then you can - how did that old song go? - take that job and shove it.
So why even bring it up? Jocasta here doesn't need to be guarded unless Jocasta (meaning, her player) decides she wants to be. And if you aren't interested in playing bodyguard, then you have no business forcing the player to stay at home.
Sure it would. In the end (and if it comes down to it; ideally, it doesn't), as a more ordinary warrior or scout (or whatever my function is) I'm far more expendable than is the only one of something that any of us will ever see again. Your bodyguard PC is a perfect example: in that role, if things get desperate, my job as bodyguard is to take that metaphorical bullet and sacrifice myself so that the last mage can survive.
This makes no sense. Just because there are a million warriors doesn't mean they're expendable. They're not nameless NPCs; they're player characters. Unless the people at your table care so little about their characters that they don't bother to come up with any sort of characterization or background for them. There's bunches of chaotic neutral tiefling rogues out there, but there's only one Rime.
Also, you're completely misunderstanding my example. In my example, two players would mutually decide that one of them would act/be hired as the other's guard. The bodyguard's
character may think that they're more expendable, but that's only because the player is choosing to have them that way.
Some characters work well in teams, others don't. Most often it's a mix. Betrayal, double-crossing, hidden agendae, or just pure chaos (benign or otherwise) - it's all fair game.
It's poor teamwork, and there's no reason why those characters would work together if they couldn't trust each other. The only reason why you
do work together is meta-reasons, because most RPGs are for groups of 3-5.