Okay so Grian's original skin was literally just link
You can see it in Evo.
He can also time travel, like Link Then there's Skyward Sword, where the hylians bonded with Loftwings- giant birds- and lived in the sky Therefore From a lore standpoint Grian could totally be a Link.
There's also Wynncraft, which he was an admin of. Where he used magic wands, y'know, like in LOZ most of the time when you use magic.
Link has also gone to alternate worlds before, such as in Twilight Princess, and also in Smash Bros, another point towards this.
Now, this means something great:
'Evil' Grian is just Dark Link who hasn't had a good fight in a while and is down to cause some chaos. Or, both have merged into one. Who knows at this point.
Now, even better.
Think about this In a couple of the games, Link gets split into 4 links: Red, Vio, Green, and Blue. So by that logic, Grian is either Red, or, he can split himself into 4 gremlins. Which could cause a lot of chaos.
Vio is shown to be the logical and analytical side, who doesn't hesitate to fake changing sides, and actually has done so. He’s the part that understands redstone to an extent, and will use that knowledge for traps. Green is the leader, the one who keeps everyone safe, and would kill if needed, though prefers not to. Red is the remains of childish innocence and is shown to be the magic specialist, especially fire magic. Blue is the angry one. The one who wants to break and wreck things. He's shown to use both bombs and a hammer. Perhaps a Ban Hammer from Grian's time as an admin?
There was also some definite sexual tension at the least between Shadow Link and Vio, so take from that as you will. (This gives a chance for disaster bi Grian)