He might just BE the dragon. Dragons help knights. Tails helps Sonic. Instant connection! Or he could be Aurthur who's a fraidy but gains courage at the end after he and Sonic defeat the black knight and acquires Caliber when Sonic leaves to his own world.
Eh, I dunno.
I'm thinking that, if they intend to do more to stay to Arthurian mythos, the "Arthur" that is masquerading as the Black Knight is probably Mordred (or an accomplice), while the REAL Arthur is Calibur, the sword. Probably was turned into the sword upon Mordred getting the upper hand.
It's simple and yet contrived enough, that I bet Sega would use such an angle. Sonic is not known for Pulitzer-material in their storylines, after all.
As for Tails' role? If he's a knight, then he'll probably be Galahad, the Knight that represented "Purity" and all that good stuff.