There's room in the stat window, could you put immunities there? Or do you want us to discover them ourselves?
Kira
5 will, rest skill
Protect and Stun
Joanne
Full offense
Magic Sparks
Thalia
Full offense
Throwing Knives -> Backstab -> Sudden Strike
Probably the strongest party so far.
I have been feeling for a long time, but not quite been able to put into words, that while this damage system is easy to understand it promotes all-or-nothing builds. I've never had someone get "a little strength" or "a little accuracy", it's always all or nothing. And given how rewarding it is at least with these low numbers I don't see myself changing that mentality unless I'm already oneshotting everything and certain I can keep doing that (and then I'd probably take speed lol), or I just cannot possibly survive without taking defensive stats.
For what it's worth I'm still not really quite able to put this into words.
With larger numbers, say 60 str 60 acc, if I took another point in a damage stat I'd only get a ~1.67% damage increase. But at small numbers with like 10/10, another damage point is worth +10% damage (the % increase is the same no matter the enemy's defense) and I'd be a fool to refuse. Maybe there's a way to keep the swings from being so dramatic without using larger values or a trickier system. It actually works for defense too though - going from 1 to 2 defense halves your damage taken, but going from 19 to 20 isn't anywhere near as significant.
Also I'm very skeptical about increasing health, even on Kira. For 1 point I get... 2 health. Is +1 defense going to effectively help much more? Surely. Until everything hits Kira for 1, but that just doesn't happen. And if it does then I'll just get speed or SP. Sure health improves heals, but it also improves DoT's.
I think enemies could use some raw damage nukes, unaffected by strength, accuracy and defense. This would mean defense stacking isn't the easily best solution to any survivability problem as far as stats go, and also you could make it pierce Protect so enemies could use it on Joanne and Thalia. Since it's not affected by defense you don't need to figure out how to make it dangerous to high defense Kira without making it oneshot her sisters.
Hell you could even give the sisters raw damage nukes too (make them ignore acc/str/def but scale with level). At least it would let a 0 offense Kira still do damage. She actually doesn't tend to reach 1 damage hits unlike a 0 offense Joanne does, but I think Kira just starts with more offense or has more on gear. Anyway Strike Through and Wide Slash do almost the same thing so there's room for it.
Back to the previous point though, +2 health for 1 point is too weak.
Oh and lastly. Tornado hits 3 targets

and are you for real, Invisibility +3% to all resistances? I'm almost sorry for calling Bramble Coat bad. I thought it would be +3 to your defense stat against elemental attacks.