What to do with a +def/- spd spring catria

Hey, my handle is usually Yoshi =) I think her best role is nerfing the other team anyway, so you should commit to that. Make her seal a buff or team boost (def tactic), keep draw back if you need the movement aid, otherwise change that to reciprocal aid. For your A slot...probably Triangle Adept, with Fury a distant second (bc she needs to stay >50% hp).
The idea is that she should be chilling in the back, with her weapon and B slot debuffs active, and within two spaces of the units that need to benefit from her drive speed. She should only swoop in to make a kill if there's no one else left to reciprocate.

Basically think of her like Gunnthara; she's not going to carry that much weight by herself, but when surrounded by a great time she's going to make them even better, with her debuffs giving them a critical edge.