Alright, finally weighing in with my opinion.
First, Avril. I say rank both. I hadn't really thought of that before, and was ready to deal with an annoying votesplit, but ranking Avril and Ice Queen separately sounds like a feasible solution, and even has a precedent with Fei and Id. Granted, they don't even look different unlike Fei and Id, but it's the same concept in principle, so I'd support it all the way. This leaves Mountain Medium or not as the only votesplit on Avril, and a fairly small one (mid-high light vs. low middle).
I'd also support Volsung, though again, that's a case of not allowing double act chains since I've never seen him do it. Granted, it wasn't exactly a long fight. I think he can use Double Action + Readying Sword + GZ Nemesis, then use Double Action on his next turn, move to Gram-Zanber, Readying Sword, GZ Nemesis all within that double act? Pretty nasty. Not holding them to taking the turns to start using moves probably helps >_>
Preliminary ranking opinions:
Dean: He's got... okay HP, decent Def and Res, not too fail Rfx, mehish damage. Alternately, not taken against Mediums (which is what I think I'll do) he's got pretty much average HP, Rfx, and damage, with a speed option in the XERD_003SS or Mithral Guard, and uh... Double Critical for healers? Light, not a good or bad one.
Rebecca: Averagish HP, Def, crappy Res, decent Rfx, about average damage. She can get some speed if she doesn't mind sacrificing some Def, and Continuous Attack might come into play? Yeah, a slightly different Dean, Light.
Avril: Mountain Medium, is held against average with mediums included for now. Excellent (curve shredding!) HP and Def, with defensive buffing, (crappy) regen if you allow LP, Blocker for a further anti-phsyical game alongside the maybe useful Critical Heal. I ignore LP for damage, so she's a bit above average there. Granted, Rfx isn't so hot, but she's durable enough to handle it pretty well, blah blah can trade off Def for Rfx.. So, a decent physical slugfesting Low Middle, with minor tricks.
Greg: Sword Medium (I like giving any justifiable mediums, sue me >_>). He has the same excellent HP as Avril, with good Def but mediocre Res. Also slow, but armor. Gamebest damage by a little though, I don't give him leypoints (I think...) so it's not all that great. Might let him get one on the second turn? Hyper and Quicken could both be pretty useful, and if it's rate is good so could having Counter with a Comet Mark. Plus, Sonic Vision and Intrude. A decent Middle, maybe high. Want to know how good Hyper and Quicken are.
Carol: Luck Medium yo! HP and Def are bad with Greg and Avril's effect on average with Mediums. Gamebest Res by a longshot helps make up for it, combined with excellent speed to use her decent parasitic attack Life Drain. Fragile and Slow Down both aid her game a fair bit, and having a 25 FP bonus to work with combined with all of that might see her to a Chapapanga. An okay Middle, really held back by the HP and only healing 25% with Life Drain.
Chuck: Moon Medium fun. Fast, though lacking HP. Def isn't so bad, though, same with Res, and all that with Status lock makes him kinda hard to kill. Damage starts off pretty average, but it's when he starts losing HP that he becomes threatening. Damage Up quickly racks up the damage if the fight doesn't end quickly, and dropping him below 25% activates the Prism Staff, giving him 25 FP which may get him to an Intrude, which is lethal with Damage Up at such low HP. The HP and low initial damage hurt, but that speed and limit-esque fighting style make him a competent Middle.
Bosses!
Nightburn: Kinda fast, with decent HP - a little ways over PC - that isn't too hurt by the lowish Def, and he has above average Res. Halves two elements, though fire weakness hurts a bit. Dual Impulse is decent 3HKO damage, and Graviton is a nice trick, since I toss turn limits. Low Heavy, though a potential downgrader in a strong pool.
IQA: Fun dueler. Fast, for one, which is kind of a WA5 thing. Decent enough HP, maybe 1.5x. She starts in that Wind LP, so I let her have it, giving her a solid 2HKO wind magic, with a passable physical and water magic. 4D Pocket autowins against most PCs, to boot, if it's anything like Belial's. If her Blocker skills are both really about 33%, she becomes a good bit harder to kill, and with the speed, PC killing, and decent damage makes a solid Low Godlike.
Note: Taking PCs against the averages listed for Fereydoon, since you can, after all, fight them all in any order.
Persephone: That good speed with Quicken? Yes please, and I allow Gatling Gun to focus since I had to feel one of those when I fought her. Only fair for the Godlikes to eat that pain >_>. With decent HP and defenses, and some bare 2HKO fire magic rounds out a very solid Godlike package. Gatling Gun... is pain...
Fereydoon: Yoinks. Even faster than Persephone, he doesn't need her Quicken spell to lap people. He also has the best HP of the Sentinels to boot, with decent enough defenses. Laser Silhouette kills PCs, and inflicts very solid damage on bosses, his speed compensating for the delay on the attack. Magic sucks, but Laser Silhouette's damage is fixed so it doesn't matter. Godlike.
Elvis: Even faster! HP is decent, with good Def (but iffy Res)! Damage, not as good. Hydro Pressure is okay water magic, and Variant Attack hurts like hell at low HP. Blocker... depends on the rate. Those elemental weaknesses are just bad though, so he's very much the anti-physical Sentinel, hating mages in general. Plus, his damage takes a while to get going, so only Heavy/Godlike borderline.
Kartikeya: Yay psycho. Not quite so fast as his fellows, though he keeps up a respectable HP total, with a defensive stat spread much like Elvis. Biggest problem? His big scary delay attack barely OHKOs, and doesn't even have fixed damage, so it fails against Mog >_>. Barely 2HKOing with Sky Twister doesn't make up for it much. The only Sentinel to be a definite Heavy, though still a good one.
Volsung: Well, the HP is certainly solid, though the defenses not so much so. Starts off with kind of mediocre speed for a WA5 boss, only a little above average. Of course, then he moves, and Double Acts into a GZ Nemesis, which kicks your ass. Then he suddenly becomes a freaky speed demon, doubling Heavy Crush for an effective solid 2HKO, Stone Bullet for an effective 1HKO earth magic, or just going and grabbing Gram-Zanber for another overkill GZ Nemesis. With my ignoring of turn limits, and allowing him to Double Act without chaining them (for now at least, if I can be convinced of it he'll just be a DNR uber-godlike), that initial shot of GZ Nemesis, and having speed and durability to fire it off, followed by even better speed with a continued flurry of damage... yeah, High Godlike under my current view of him, and a slim championship contender.