Dion is an enemy you first encounter in the Slumbering Dragon inn in Savina. He is a pawn of the Golden Demon and waited for you at the inn just to confirm that you were really alive. The Golden Demon thought you were dead but recently found out you were still alive.
Dion teleports away, but you two meet later and fight right outside the walls of Savina. He has a Limit Break attack where he disapears in a flash and seconds later attacks you from behind. You can counter by quickly pressing the number that flashes on the screen.
After getting wounded enough, Dion teleports away from the battle and you don't see him again in the game's current unfinished state.