I have a fully maxed Kerria and i have experience using her.
Like game i will make it short,too
Everything is already covered.
@Kurenai and maotd
There are in my opinion 4 Pillars of Competetive Play:
Counter, Crit, Debuff and SolarBlast.
Each of these Archetype has at least one girl which represents the brokeness of those Archetypes the best.
For Counter it's definetely Saffron, for Crit it's in my opinion Rainbow Rose (Yes i think in pure aggressive Crit Builds Rainbow Rose is better than Kerria. And lets be honest most Crit Builds are offensive ones), for SolarBlast it's Dogwood and for Debuff the absolute best girls are Aconite and Kerria (and she is not even a Debuff Girl xD).
Aconite + Kerria make an awesome Debuff Core which i like to call the "Sisters of Immortality"
Kerria got a lot of competition lately and she still reigns supreme. The only thing which annoys me slightly is her Low Speed. It's still possible to fix that issue somewhat easily compared to Lavender's or Cosmos' Speed.
So yeah Kerria has nice solid stat distribution (even tough her defense is a bit lacking) and an awesome Ability Set which synergizes well with her HP Absorb.
I mean there is a reason Kerria besides Maple is the only Girl i maxed out (5 Skill Levels and 4 Equipment Slots). Also basically everyone in this forum has Kerria by this point or has plans to get her. She is just that good!!!
Final Verdict: S-Tier