Rumia for me... moonlight ray was always powerful in lot1 but her bad stats made it so it never really seemed all that great most of the time..however in lot2 it seems even MORE powerful, AND she can bypass some defenses with her passive skill, ensuring moonlight ray is ALWAYS either decently powerful, or stupid effing op... oh, and her group heal is bigger and better and faster than reimu's was in lot1, oh and it cures debuffs too, geeze...Might be my imagination but she seems a bit less squishy too. I mean I'm not saying she has better defenses than 1 but she just doesn't seem to get randomly 1shot as often if you leave her out.
I WOULD say Aya but I actually expected her to be pretty effing amazing as soon as I saw she had an atb-filling spell... fast character + atb filling spell = more than double anyone's dps. Oh yeah, and the fact evasion works now doesn't hurt either.
nitori... now before you ask how could I not think she'd be op? Well she was pretty op in lot1 already, so I figured she'd get nerfed a bit at least... instead she got buffed?! what?! Yeah, I didn't expect THAT.
hina: Wasn't expecting her debuff move to work so frequently on enemies that resist everyone else's debuffs (except kogasa's) all the time.
As for Remilia, I still think she's kinda watered down kasen, I mean yeah the sdm buff seems to matter, and as for last fortress... I don't know, I haven't had any boss battles with more than 2 deaths yet. But I found she was much better in lot1, and nobody else really thought she was all that in lot1 but me. In lot1 spear was more powerful I found (as in enemies with extreme defenses that take 0 from attacks that were not full action bar moves like slash of eternity, koin3, megawatt, SFN, etc, would take damage from remi's spear before nearly everything else that wasn't specifically good at piercing defenses like silent selene, asteroid belt, minoruke's single target spell, etc). Her self-buff in lot1 was more significant too because there were no majesties or monk passives, no byakuren, no party-buff passives like the one for guardian...AND in lot1 it was pretty easy to make herself immune to poison and paralyze WITHOUT gimping her stats (iirc one item seemed almost made just for that, heart of badges or something? I forget the name)...Last, spear did non-elemental damage, now enemies have physical resistance, which makes her usefulness even more situational (though the odd boss that is weak to physical is obviously going to hurt)...Also, could be my imagination, but I'm pretty sure she's slower in lot2 as well. But that might just be because high speed items in lot2 are far more common like ninja mask, meteor drive, lillium's panties, etc...And none really seem to be good fits on her so much as other characters). I mean she's still fast, but she's always hit 1000 at the start of the fight while other medium-speed characters like reimu were still not even at 9000, maybe even 8000. She was FAST.
I still haven't tried many characters like kanako though.