Also in fairy tail, the ability of a certain being sort of classifies you as such. Gray is considered a Devil Slayer cause he uses the power of Devils. That's why Skullion said he doesn't eat devils. However if he was an Ice Dragon Slayer, he wouldn't say that. Cause in fairy tail having the abilities of a certain species classifies you as so. Let's look at Mirajane. She uses demonic take over, so she is considered a Devil. Maybe not a pure one in the face of Tartarus, but she's a demon. Let's look at Dimaria Yesta, she has the powers of the god of time. So, she is a god. That's why Sherria was able to affect her like that. Sherria is technically a God by way of how powers work in FT. They don't just have the abilities of such and it's left at that, but they are that, in a way. Gray is a Devil, Mira is a Devil, Natsu's a dragon, Irene's a dragon, Dimaria is a God, Sherria is a God, and Zancrow is a god. Elfman is considered a beast. Because of the animal take overs.
Slayer magic is created to be effective against whatever it is they slay. So they always have an advantage vs that thing they slay. Mirajane is not a slayer, Gray is. So devil vs devil, he has a major advantage. But they are both devils. So by default of that, should both be affected by devil slayer magic. Cause they are Devils.
