Despite her stats and her low ATK, her weapon more than makes up for it.
-First off, she has a bonus against Cavalry, which while it sounds weak for a Beast unit, Panne is a Cavalry unit herself, meaning she can make decent use of it due to her Movement.
-Second, she has a Fury Solo in her weapon, meaning that she gets a +3 to all her stats when she's alone. So when Panne fights enemies on her own when transformed, she has a ATK stat of 48. That's actually not bad and overall pretty decent. Not to mention she backs it up with 37 SPD and 33 DEF, which gives her some pretty good tankiness on the side.
-Third, when transformed, she gives a -4 Combat Debuff to ATK and DEF. So when transformed, if you factor in the above, she actually has an
ATK Stat of 52 against foes. Not only that, but that also adds gives her a DEF of 37 combined with her 37 SPD. An ATK of 52 with a SPD of 37 and a DEF of 37 is actually pretty good. Combined with the fact that foes Cannot Follow-up, meaning skills like Quick Riposte and Vengeful Fighter do not work against her and she is actually decently effective against tanks and armors.
Looking at her, Panne is actually a pretty good unit. And she can Chill SPD, meaning that with her good SPD, she can prevent some faster opponents from doubling her, adding to her Tankiness. But what will really make her shine is the Solo skills. She can make fantastic use of both ATK/SPD Solo or ATK/DEF Solo, adding to her powerful Solo unit status. She can also make great use of Tibarn's Sturdy Impact. And due to her Solo nature, she doesn't need to fear skills such as Aversa's Night or that new skill from Selkie.
Combined with the fact that she is a Cavalry unit, meaning Hone, Goad, Ward and Fortify Cavalry will work with her. That said, if buffed by Hone or Fortify, she won't have her Transformation bonus
UNLESS she is with Kaden or Selkie.
Kaden and Selkie are also Cavalry units, meaning that she can not only get a Hone/Fortify from them, but if Kaden or Selkie are the only units beside her, she will have the Buffs as well as her Transformation bonus, allowing her to reach 58 ATK and 43 SPD, which makes her pretty dangerous. Combined with the fact that Kaden also brings in Drive buffs with his weapon. So if Panne gives Kaden a Hone Buff to him, Panne can get an additional +6 to her ATK and SPD, making it 64 ATK and 49 SPD.
So yeah, Panne is deceptively powerful.
Edit: Swiggity swooty, 5 Azuras coming for the floofy.
Erureido
Tagging due to Kaden fanart.