The ending of Axel's D.D. Castle event clearly points to Supreme King being an actual, duel-able enemy in the future.
And that also brings the possibility of him becoming unlockable.
The other difference is that Marik does not have any distinct cards from the one in the game. Supreme king has his evil heros which have not appeared yet making him more likely to appear, most likely at the next Axel event.
Anonymous
MangakaJ96
