|When I first saw Valentines Roy was a cavalry bow, I immediately joked about him being diet Lyn. This set comes from having thought further on it, and taking that joke to a bit of an extreme by recreating Lyn's default kit as closely as possible with Roy.
First of all, we stick with the default weapon, because it actually imitates Brave Lyn's bow pretty nicely. You trade some base damage for ignoring buffs from everyone, not just mages. With a refinement, you also trade one speed for two HP, which isn't too bad.
Draconic Aura is the same as what Lyn regularly has, and also will work perfectly fine with Roy, thanks to his high base Attack. If you wanted to, you could use Iceberg as well.
Swift Sparrow 2 is the same thing as what Lyn has, but due to how we're replicating Lyn, it also plays a role in assisting Roy with Windsweep's effect. If you don't have a Swift Sparrow to spare, or are more concerned about guaranteeing Windsweep activates, Darting Blow 3 is a decent alternative that isn't too hard to get.
Windsweep 3 is our way of imitating Sacae's Blessing - while Roy's speed is low enough that he isn't too bothered losing a second hit, it does mean we really have to focus on his speed to get it to activate reliably. However, with a +Spd nature, Speed refinement, Swift Sparrow, and Phantom Spd 3, we can get his Speed effectively up to 38 - this just about guarantees that Windsweep will activate on everyone who you'd be worried about retaliating against you. And we're not even considering Cavalry buffs.
The last slot is effectively free, but since we have a theme, I've gone with what you'd probably see on a Brave Lyn - Atk Smoke 3 or Hone Cavalry. However, you can use whatever is most useful for you here.
Now, unfortunately, this build has a lot of investment to even get close to what Lyn can do out of the box, but it's not all bad - Roy does actually have some upsides with this set. His combination of abilities lets him deal with enemy bows more easily, even ones who carry Bowbreaker 3, which can be helpful. Daggers are also covered, but they're not as common.