This is an old thread by now, but now that I have leveled my Iris to 55, and gotten her to about the same level of my Lilly (weapon upgraded to +7, Soulstones at level5, decent enough Akasha and Brooches) I got some more experiences to share. If anything, I would say my Iris is slightly more optimized, since by now I got more experience at the game and I never completely bothered to fix all of the flaws my Lilly has. They have very similar stats, Iris has a lot more in Cooldown and SF-cost reduction since I emphasized on that but Lilly has, unsurprisingly, much more Crit Dmg. Crit% is for both the same, though.
First, I think the whole deal about Lilly being significantly weaker than the other characters is complete bogus. I expected a huge difference as soon as my Iris hit Lvl55 due to all the talk here in the forums, as I already had most of her equipment prepared beforehand, and surely, farming the 100% in the last Ruin Fortress areas seems a lot more efficient with Iris thanks to the high area coverage of her Ultimate skill and Bosses also go down a tad faster (though that comparison is hardly fair cause I have Big Plastika on Iris).
When it comes to raids, though, and most notably Golden Citadel or Steel Prison Lvl15 (the solo dungeon), I see Iris' flaws, closing the Gap between Lilly and Iris. First of all, not being able to use items is a HUGE setback for Iris, not being able to heal is one thing and can easily be remedied with a Stella on your team, but well, having the need of a Stella is a problem of her class that Lilly simply does not have - and you are on your own in Steel Prison anyways. How well you fare on your own is the best, if not only way to compare the strength of an individual class in the first place.
What hurts her even more, however, is the inability to use SF Limos. Iris' DPS greatly depends on her ability to spam skills, and the lack of SF recovery items puts a HUGE dent in that. Her innate recovery is awful, her basic attack combo is horrible and her Soulstrike as the only means to replenish it has the same problem as most of her other skills which I will elaborate further later, leaving her pretty much helpless if her Soulforce ever runs low, and with enough enemies to swarm her, that can happen quite easily.
Another problem she has that Lilly doesn't is the commitment that comes when using her skills. Her Fire Somersault is the one exception, but almost all of the other skills require her to stand fixed in a place for quite some time, or greatly reduce her mobility. In the Lvl14 Steel Prison levels with the Taifons, you can basically not use a single skill without getting hit, and you have no way to heal. You also barely can hit Rawan without taking hits from him and the same goes for when small enemies with ranged attacks are around. I mean sure, these still arent that big of a deal, but the fact stands SP is easier with Lilly and if there is future content with these mechanics, Iris WILL struggle there, while Lilly will not.
I also finish SP much faster with Lilly. When I enter SP lvl15 with Lilly, I do one tornado and the first wave of mobs is dead, I do triad and the second is dead, I do Guillotine for the third and when the 4th and fifth comes, Tornado and Triad are ready again - without having any SF problems, I can just standard combo the elite mob while Tornado kills him and get my SF back to 100%. Playing as Iris, I need to combine at least two skills for the same result, the only Iris skills that are on par with Lillys Tornado in terms of damage are her Ultimate and the one where she rides on her laser cannon, the latter which takes 10 seconds where I can't do anything else while Tornado kills the elite mob almost instantly. I don't think these skills are individually even any stronger than Lilly's, it's the fact you can mass-spam them as Iris that she gets a higher DPS.
The important thing is, I can use these skills without it being any commitment or hindrance to Lillys basic performance, you spawn Tornado, it takes half a second and I can go back to whatever else I am doing, Triad is 1 second and Guillotine is 3 seconds. When I use even Plasma fire with iris, I commit myself to standing in one spot for 3-5 seconds to shoot my bullets, I can do nothing else at that time, and if I notice i messed up and the elite mob comes to attack me before I break his Super Armor, I evade out of it and have practically wasted the move, its SF cost and the 4secs of its casting time. It is even worse with the drill, the plasma sword, eruption or the laser, who all take about 10 seconds of commitment and cost much more SF. Even her Soulstrike takes about one and a half second of commitment and likely earns you a few hits when you try to replenish SF.
This has consequences for a few things mentioned in this thread. Some people have asked for Lilly to be able to cancel out of Triad, but HELL NAWH! That one second of commitment is a minor issue compared to other classes and must by all means stay as it is for there to be at least the slightest bit of challenge when playing Lilly, changing that will just make playing her more boring. Especially considering what range it has.
The other thing is her Ultimate. Why indeed, Lillys Ultimate pales in comparison to Iris' both in terms of area covered and damage (though I'm not sure if this isnt just due to the fact I always use Iris' damage buff right beforehand). Still though, Iris' ultimate doesn't have any less of the 'enemies can just walk out of it' problem, and again: The skill comes with much higher commitment for Iris. Iris' Ultimate is an ultimate in every sense of the word, it's a room cleaner and responsible for a good deal of her overall damage output, especially when used with the buff beforehand (hell I don't know why you would ever use it without it). Using both together also costs you a majority of your SF. Therefore, it must be used very carefully. If an Elite mob survives or walks out of or if you have misplaced ur Ultimate with iris, you are actually in deep trouble since you barely have any SF left to cope; if you don't hit all the small fries, its just plain annoying and it takes ages to clean the rest up. Same thing happen to Lillys Guillotine, though, who cares? Its damage is a nice bonus at best, and being out of SF is just a regular occurence for Lilly. You can easily get it back with just a few standard combo hits.
Someone did mention how good the standard combo is doesnt matter this late in the game - yes, Im still not over it - but I beg to differ here. It's especially in the lategame where that difference is a huge deal, since it's there where areas with forbidden item usage exist, and matter. I wouldnt say its the most important thing in the game right now, but this isnt unlikely to become more and more relevant with future content, and therefore should be kept in mind.
To sum things up, based on my own experience in the game, I consider the general consensus of this forum that Lilly is considerably weaker than other characters a mere superstition. I can only compare Iris and Lilly, but if Iris is considered the strongest and Lilly the weakest, I doubt it's any different with the other characters. What's more, I can't speak for the world boss or PVP since I haven't played either with Iris, she might very well have an edge over Lilly there, though I wouldn't count on it because what I mentioned about her SF management should apply vs. Gronko as well, and with the amount of commitment Iris' skills have, how would you even hit ANYTHING with a brain in PVP.
To conclude, I wouldnt buff Lillys skills too much, especially not in ways that increase their commitment. Rather have them work individually like I suggested before
I still don't mind the general changes like increasing the SF-gain of her basic attacks or making her scale better (seriously, Jin can just do 1-3 hits and his SF is back to max again). And I especially like the idea of her group heal now, it would be good to have 2/6 characters with healing abilities and not just make Stella mandatory for ANY group that wants to go into a no items dungeon.