I got a +def -spd what build can i make for her or fodder her for someone else?
She isn't very fast to start with so you aren't missing much.
Deirdre is a mage tank basically, she comes with Quick Riposte 3, so just give her Fierce Stance or Mirror Stance in her A slot if you have it, if not just give her Atk+3 in her A slot and her SS. And leave her Spd Ploy there.
Or give her Death Blow to deal with Nowi, Corrin, and M!Kana.
She's not getting much use out of that defense boon and speed is also a superbane so those certainly aren't very promising IVs.
Then again, as almost fully F2P I've put worse IVs to use so if she's your only one you can have her still tank mages and smite dragons quite well, even though that QR3 and Speed Ploy are both desirable skill fodder.
Give her an Iceberg or Glacies, a positioning support and an A skill you can spare (Mirror Stance would work pretty good on enemy phase, bond skills when next to allies, DB to possibly KO dragons, Fury... drops her quicker out of QR range so I'd skip that) and she should do pretty well.
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