On why does Oric want the Target Dead?: He came up with a genius plan: use HIS cohorts as batteries! A tense alliance was formed between the Devils and the humans.
The Thief was trusted with an unbelievably powerful Soul Star. It granted him dominance over the Devil and ALL her sinister siblings. He could manage them, tell them exactly what they needed to do to run the place HIS way, under threat of retaliation. There were three such Soul Stars, but that's another story.
However... after the first run of his 'experiment', the Devil told him: they know what they need to do, meaning the Thief himself was no longer needed. Keeping him alive was a useless power drain for no return. He'd been fired.
The Thief offered her a trade: use the soul power my system grants you to do as I please, and I'll keep you alive. Thus the Last Devil watched over (and became the conduit for) ALL souls passing through her once-powerful dungeon.
The Thief took her minions - her cherubs - as tribute. Just as allegedly immortal as their elders, the cherubs helped watch over the dungeon, ensuring all went according to the Thief's plans. None dared to face Oric's Hammer. The end.
The Target: You don't know a lot about him either. He changes the most. You only know the others are supposed to kill him. However, you're starting to think this is not true because he is NEVER dead when he reaches you. Oh well. You don't believe in fate much.
The Thief: You hate this person. He is the one who originally took your Soul Star and imprisoned you here for defying him. You actually aren't sure what you did to anger him so much, but maybe it had to do with killing people? Brutal, selfish murder usually makes people mad.
Do note that, during the Al intermission, Al specifically complains about Illborn attacking him for reasons unexplained instead of the villagers. The villagers - or at least Chief - are also illborn. This indicates that perhaps some are more monstrous/hostile than others. However, one thing no one/not many noticed is that Ciro and Riltia have been constantly assaulted by Illborn throughout the game. Illborn want the Soul Charge Ciro has vast quantities of and, somehow, by killing him, they'd get to it.
Warrens has an overarching theme: the separation of Body and Soul. In real life, the Soul is an abstract, philosophical concept at best, but in the Warrens, each person's soul is an actual object that takes up physical space in the dungeon underworld.
As Yaos showed us, losing Soul Charge alone causes characters to bleed from the mouth/eyes/face/elsewhere. When Yaos and Riltia ran out of Soul Charge, they transformed into horrible monsters briefly referred to as "Willborn." We do not know what Willborn means yet except it's a variation of Illborn created from the following circumstances
Long after our revival, we found the Asynchrony Catalyst in the Last Seraphim's Sanctuary. This, along with Slog's revival via Influx, means we were revived because the Asynchrony Catalyst was in the slot.

Al implies he didn't do anything but put Ciro in the tube, and he doesn't know how the revival happened. The Asynchrony Catalyst was already there when he arrived.
...So why was it there in the first place?
From all this, we can discern one thing: the link is stronger in the Soul->Body direction. Somehow, the LSS revives the bodies of dead characters but only when a Soul Star is available. Characters deteriorate (suffer internal bleeding) when their Soul Star loses power. When Soul Charge is completely depleted, characters become Illborn or worse.
We don't know why this happens yet, but we know one thing: in Warrens, your body is merely a hollow shell created (and potentially ruined) by your soul. Everything "you" are is in your Soul, and your soul retains its form across class changes, gender swaps, and revivals. Everything else - your body, powers, opinions - is perfectly malleable and irrelevant.






OK, the observant among you: One of these quotes is not like the others.
Which one? The one where FFS revealed the nature of illborn. What does it show? When you kill someone, you get their soul charge.
Oric's system is this:
He controls the Last Seraphim, the Lord, by keeping her servants, the Cherubs, captive. He uses his own minions, the Siblings, to keep the Cherubs in line and act as batteries. Batteries for soul charge. Which powers the warrens, according to Cains story. Al's quote reveals that Cherish is being used as a battery, too, for powering the Underside (Hints: Orange stuff revived us, Cain called Cherish "the orange one").
This energy powers the levels directly from the soul stars, and the siblings can only hold so much charge (too much, they "oversoul", wasting the charge). By killing and resurrecting someone with charge, they refill their batteries, and keep power to the level. The reason Cain kills anyone who steps in his circle is to take their charge and keep his level powered. Higher siblings on the totem pole, deeper in the dungeon, meet Target less, but Target has more soul charge when he/she meets them, more charge less often. They kill him/her and take the charge, and keep the warrens powered.
Cain implies in the Al quote that he got resurrected; that all the siblings resurrect when they die. Even Target. Especially Target. Target is the one who changes the most, which with the second to last quote, copied here:
Everything "you" are is in your Soul, and your soul retains its form across class changes, gender swaps, and revivals. Everything else - your body, powers, opinions - is perfectly malleable and irrelevant.
means that when you revive, you keep the same soul but not the same body; you change whenever you die. Therefore, Target dies the most. So Oric can afford to kill Target over and over, and it doesn't waste any soul charge. In fact, it only generates it. More and more of it.
What I'm saying is, that Oric wants Target dead, by a sibling's hand specifically, so that the soul charge he/she accumulates can go to keeping the Warrens alive. While I was gathering this, I thought, The Devil Herself tried to betray the King they set up. Betweenford was their agent for setting up Oric as King. Newford wants to set us up as King/Queen. The last Devil (Seraphim) is Newford's boss, and Newford is therefor her agent for setting up Ciro as King. Therefor, the Last Seraphim is planning on betraying us.