A wild Gray appeared!

I’d recommend not using him at all, as he’s mainly useful for helping other units train because of his Sword Valor skill. (Is it Sword Valor? I have no idea. I don’t have him) If you want to use him, Bonfire would be nice. If you aren’t F2P a brazen could work. You could use Wrath if you won’t use Vantage. QR seal for doubles. Odd Atk Wave for even more power or Even Spd Wave for a boost in it. This build is probably terrible because I’m not great with builds, but I hope it helps. :D

The Zanbato refine paired with his wind boost keeps him pretty quick if youre careful. For sacred seals, health buffs (for the boost) or just more speed are easy fixes. Quick riposte is a great idea for sure, but another option is running renewal and sol to keep his health up for wind boost. May not be ideal for arena, but in chain challenges and tempest trials can keep him relevant.

Honestly, I think you should make him a tank, even if he can't double. Zanbato is perfect anti-Calvary and Gray has some desent strength to use it with just one hit. Just put some enemy phase skills on him like Steady Stance or Breath and maybe a Chill or something and he should preform pretty well with a Bonfire set up. I could also recommend a Brazen/Vengeance build. Cause you need to lower your health for Brazen to activate and Vengeance synergizes perfect with that. If you go with that, I recommend Brazen Atk/Spd or /Def.
But if you think he's too much investment, then I say fodder Zanbato. It's a pretty rare sword and can be pretty useful at times.

Hey I also got a +def -spd Gray on the free summon round. Coincidence?