What to do with a terrible IV B.Roy that pity broke me on Muspell summoning?

The only thing that is worth Foddering him off is Galeforce but we have Cordelia for that but If you don't want to waste 20K feathers then sure fodder him off.
BUT
if you don't like the idea of foddering him off then give him Fury and he should be ok, With Horse emblem buffs he won't struggle killing.

Do what you feel is best, you could give desperation 3 aaaand galeforce to a unit, but I'd keep it for a merge

I'd say fodder him off. B!Roy's statline is already pretty bad in comparison to all the other Sword Cavs, but -Spd is the final nail in the coffin. With an offensive spread of 32/31, he'll find it extremely hard to kill stuff

Heavy blade on a 19 MGT sword.
Brave Roy is one of the best galeforce cavalier.
Others red cavalier (siegbert, swordhart, sigurd, ares...) are great too, but B-Roy is designed to be this galeforce bro

-atk hurts way more than -spd. You should try it and keep him for more merge (and better IVs)

I have the same IV on him. I went with a completely budget way and gave him LnD. It works decently enough, but he desperately needs hone cavalry or any other type of +6 buffs to his offensive stats. He does get the job done, but sadly, he's heavily outclassed by all the newer sword cavaliers.