So, I randomly got the idea while finishing up the P4 topic (still in progress, but big additions made recently) for P4 characters. Probably a biiiiiiiiiiit too early, but why not?
While adding a support character is cool...just won't work as well in this vein, so no Rise hype ;_;
The best people to add...probably Yosuke, Chie, Naoto, Teddie/Yukiko (Teddie is more interesting, really, with the support spells, but...could see Yukiko), Soji. Kanji is...a bit 1-D, though Matarukaja is nice. To limit it to 3 people...probably Yosuke, Naoto, Soji (if no Soji, Teddie).
Yosuke - Overall balanced. Can heal decently early, dispel, MT status, MT damage, fast, decent durability, etc. Wind isn't a common element either, so not really walled, and he resists fire once he upgrades. Electric weakness early hurts, but...well, see far below.
Naoto - No weaknesses, and ends up arguably immune to ID, and resists fire. Is a bit below average durability, but speed is good, and she would get Algernon early. Almighty damage is nice to avoid resistances. MT decent ID is helpful, though her damage is overall...eh. Mind Charge helps, but is two turns.
Teddie - Electric weakness sucks, but he has Mataru and Maraku kaja, with lots of full healing and status healing (though not at once like Yukiko), and at least guards ice! Damage is good, though ice, which sucks. Basically, a variation on a healer, though depending on how you view S.Links...
Yukiko - Ice weakness sucks, but better resistances than Teddie. More SP, slightly less durable, slightly slower, slightly more damaging...but gets Salvation, and has a neat stock of weapons. Lacks support skills, though. Pretty equal, overall.
Chie - Has speed, and is a decent ice mage and physical clearer early on. Sadly, gets a HUGE downspike from around 30-50, and the ice magic dies off around here as well. Once she gets Power Charge, she really picks up, but it really takes level 70 to get her damage fun going...not...really ideal for the dungeon unless you allow the S.Link skills...but probably not good for the dungeon. Skull Cracker is nice, though.
Kanji - Well, he's a tank and his physical damage is awesome. And he has Matarukaja and Rakukaja, which are decent. A bit slow, and his electric damage...really could be better, though it's not completely awful. Eh...better than Chie, probably?
Soji - Ok, so, Floor 1 is start of the game, Floor 7 is endgame? So about 10-12 levels each floor? Ok, I'll get back to working on his set-up, but that basically means he won't get Yoshi until endgame, but then gets Helel and others. Also has all his unique accessories, which are helpful. You might want to use/not use the "Team loses if he dies", and adjust accordingly (fewer points if the former, more if the latter).
So...mmm...Yosuke is the best choice here (decent for anything, no Soji interps), probably followed by...uh...Naoto or Teddie/Yukiko. Kanji and Chie aren't much special, and Chie can be a weak link for 2-3 floors.