Yeah! It's finally time for Kerria! I'll try to be objective (but anyway, even if I'm subjective it will be the same).

Final verdict:
She is S tier.
What? I need to explain? But why?


Stats:
HP: She has good HP. Not awesome but enough to be over 20k. It's the minimum needed. She would be even better with more HP but she can't be 100% perfect or she will break the game.
ATK: Really good ATK too but still a bit far from the top. Hopefully she has solid assets to "compensate" that.
DEF: Her lowest stat and she is close to the botom. On one side, it's not a good thing. She is one of the rare girls who needs all the stats. But on the other side, it's better like that. ATK and HP are too important for her.
I never speak of speed but this time I will. She is a bit slow with only 430 speed. It's not that important but it will restrain you in team building and forbid some good allies if you want to stay in a decent speed range.

Skill
x4.3 damage on 1 target + life steal with 38% activation rate.
That's pretty common life steal skill but that's her ultimate weapon. The 38% is a bit low. It's the common for this kind of skill but on her, it's really too slow. She absolutly need her skill to be what she is.

Abilities
+40% ATK for her and 2 allies
Usually, ATK boost for more than one and less than the full team is not good. But for her, it's not a problem. You just want to buff Kerria's ATK. The other two allies are just a little bonus. They will die quickly anyway. It's a very welcome ability since Kerria needs as much ATK as possible to make her job.

+40% DEf for her and 2 allies, increase damage reduction from defending by 6.8 and Guts
Some people think it's not a good ability for her. That's not totally false since it's not an offensive skill and she is not a counter girl. But she has Guts and a better defense. Despite her ridiculous destruction power, Kerria is more defensive than offensive for me. She is closer to girls Saffron than every Glass Nukes. It's clearly her less usefull ability but it's still a solid option on her.

Increases your Damage to enemies by Current HP / Max HP / 2.
And that's her first toy that make her broken. It increase your damages by 50% at full life. And maybe more if the "Max HP" in the math take the actual Max HP of the girl and not the Max HP after the Over Limit bonus.
It's an awesome ability but it's terribly double edged. Your damages will be pitiful at low life, when you need to hit as hard as possible to recover your precious HP.
A variant of it where your damages increase when your HP are low would be far better for her but Kerria would be unbeatable.

80% evasion on turn 1 and 2, 50% after
And her second toy and the actual reason of her brokeness. Evasion on this girl is just perfect. She hits very hard, she can recover her HP with life steal and pests have only 50% chances to hit her (unless it's some unfair pests with accuracy buff). With that, she will not need to activate her skill every turn to stay alive. Okay. she will need to activate evasion or her skill to stay alive and bad RNG is her nemesis. but ith the right team and a lack of awful luck, she will be able to survive everything.

Team Build
She is a special kind of girl. You will not use Kerria in a team. You will build a team around her. If you want to use her, she will be your team pillar.
I'm not a specialist on Kerria's team building since I never get the right 6* to build around her.
I guess ther better archetype for her is damage reduction team. She needs to take as less damages as possible and damage reduction teams are perfect for her. But due to her low skill rate and lack of self skill activation, she will needs a lot of skill act.
When I skeap about that, two girls comes in my mind:
Oncidium Bride with 15% damages reduction and x1.2 skill act
Lavender Valentine with the same assets.
They also bring skill damage buff and damage buff on bosses. But Lavender is really slow and will sink your teal speed for ever with Kerria around.
If you seach on the promoted girls side, there is Aconite with the same kind of abilities (but better) and the wonderful White Tulip with skill act, damage reduction AND 12% precision reduction.

Or you can use a more offensive version of the team with a lot of skill act, a lot of damage BOOST and a lot of Crit. In that kind of archetype, she will murder everything but she will be a bit more fragile.

Or you can put her in any random team. She will not reach her full potential but she is strong enough to do good things by herself with just a few ATK buff to support her.

Conclusion
Kerria is one of the strongest girls and for me (and I4m pretty sure for a lot of people her), she is one of the FKG Triforce pieces. Her abilities are pure diamond, her stats are well balanced and her skill is the cerry over all the other cherries on the Kerria's cake.
Team building wise, she can go everywhere, even alone and she will do her job. Fun fact, I used her in my Nutaku's raid boss team a long time ago and I finally banned her because she kills every regular bosses. She can even handle easily the 1st phase raid boss event's raid boss and hit the turn limit... with a random team full of not bloomed rainbows).

Final verdict:
She is S Tier, she definite the S Tier with her friends and she will stay one of the bests girls in FKG history for a long time (even if some promoted girls tries to reach her level). I mean, just look at Kurenai's pictures. They speak by themselves. Kerria is boss level for pests bosses themself.

Personnal Feeling:
Kerria was my first Nutaku 6* and my first 6* ever, outcome from a free 100 Nutaku Gold roll (you know, those 100 free gold you got if you invite a "friend" called yourself with another account). She was better and better over the time and she is still my best girl over my two account (Nutaku and DMM), only followed by Pink Miko in the 5* class.
Maybe I'm not that objective with her and I know she has some flaws (espacially her weakness to RNJesus will) but even if I put all my subjectivity here, it will not affect her rating at all.

Now, Kerria is done. I can die in peace (but I still need to get her when I will have enough R-medals).