On the difference between fiends and the unsent:
“The dead need guidance. Filled with grief over their own death, they refuse to face their fate. They yearn to live on, and resent those still alive. You see, they envy the living. And in time, that envy turns to anger, even hate. Should these souls remain in Spira, they become fiends that prey on the living.”
But the unsent can, presumably, also become fiends or at least gain fiendlike qualities: Seymour and Yunalesca transform quite dramatically when they’re threatened. And only a subset of the group of people who aren’t sent become “the unsent” stricto sensu; the rest go straight down the fiend route.
Auron (who should know what it’s like, all things considered) says of Seymour, “His attachment to this world kept him from the next.” So is the difference between fiends and the unsent merely that the unsent have more willpower? They resist death longer, because they have a certain “attachment to this world”, a certain task they remain determined to complete, but is the implication that if they let their bitterness overtake them, they will also become fiends eventually – or, as in Ginnem’s case, remain in human form, but lose their humanity? Omega also seems to be relevant here: his “loathing of Yevon has turned him into a fiend.” Is Auron forced to remain a ray of sunshine (ha. Ha ha.) because otherwise a similar fate will come for him?
This flowchart summarises my understanding of how death, sending, and becoming fiends/unsent work:
The “unfinished business of critical importance” stage is specifically there for Jyscal Guado who defies all logical explanation. “I would think that he was sent once,” says Lulu, “but he stayed on Spira. Something, a powerful emotion, could have bound him to this world.” So he was sent, but somehow didn’t make it to the Farplane. However, when Auron is sent he seems to end up on the Farplane immediately (given that he’s in the hi-five FMV). Is Jyscal just taking longer to get there, perhaps because he “died an unclean death”? Is he not quite settled in by the time Yuna visits, allowing him to slip out into the living world? Or is there a chance that any unsent (perhaps especially those who “died an unclean death”) could escape the Farplane and come back in some form? This is brushing up against post-FFX-2-content territory here, and I need to continue my more thorough reading of the novel to try and make some sense out of this; however, those who are “beckoned” appear to act like humans as far as I’m aware, whereas Jyscal is basically a zombie, albeit one that has the wherewithal to maintain possession of a sphere he took shortly before he died, and pass it on to someone who may be able to act on its message.
What effect do sendings have on an unsent? Auron does some Suffering (what’s that? An excuse to link to this gifset? How terribly inconvenient) when Yuna sends Jyscal, but doesn’t seem to be affected when Ginnem, Yunalesca, and Seymour are sent later on. Is it simply that he’s not expecting to have any trouble the first time it happens, and knows to watch out on later occasions? Those unsent who don’t want to be sent seem to need to be weakened first: again, see Seymour and Yunalesca, plus Ginnem, although Yojimbo kind of stands in for her here.
Auron, Belgemine, and Mika all end up quite willing to be sent once they have either done or perhaps ineluctably failed their principal task. Jyscal also seems to be sent without any trouble, implying that his principal task is just the one act of passing on his sphere and tipping off the world at large about Seymour’s character. Belgemine’s principal task is to train a summoner who can be sure of defeating Sin (let’s face it, if you grind enough to get to the Magus Sisters you’re not going to have any difficulty with Braska’s Final Aeon). Auron’s principal task appears to have been bringing Tidus to Spira and getting him to kill his dad – not merely watching over him in Zanarkand, but specifically the latter part:
“Then I went to Zanarkand … where I watched over you. So that one day I could bring you to Spira.”
But if his task was literally no more than to “bring [Tidus] to Spira”, he would have been done as soon as Tidus got to Baaj. My hypothesis, therefore, is that Jecht specifically asked Auron to use Tidus to break the cycle, presumably at some point after the farewell scene we see in Zanarkand (because all he says there is “take care of my son”). What I’m not sure about is how much of Auron’s principal task comes from himself (the desire to avenge his, cough, friends, and to see Sin gone for good) and how much of it comes from Jecht (bringing Tidus to Spira and setting his “story” in motion).
Mika’s principal task, I guess, was ensuring that Sin endured forever, and once he heard that Yunalesca had been disposed of he realised this was no longer possible and sent himself, apparently, out of sheer desperation. I’m not sure how he managed to do this, given that he is not a summoner, if we believe an NPC in Luca who tells us that “Throughout the long history of Yevon, this is the first time a summoner [i.e. Seymour] has ever been appointed maester.” Maybe all unsent have the power to send themselves once their task is complete, and Auron and Belgemine just wait until Yuna sends them instead because they know she’s about to do it and they don’t have to bother?
The unsent don’t seem to have any physical differences from the living, other than that they’re made of pyreflies and this is occasionally visible (see gifset linked above). Auron is able to conjure his own memories on command; Maechen does the same in FFX-2 although it’s implied to be somewhat accidental. Auron also explicitly notes that being dead has allowed him “to ride Sin, and go to [Dream] Zanarkand.” Other than that, he seems the same as any other person in that he’s affected the same way by battle wounds/status ailments etc. He does mention being tired more than once; I have a wee headcanon that being unsent can cause some form of perpetual fatigue, although this is mostly just an excuse to make Auron suffer more, obviously.