The fact that I've seen the least amount of theories around RK being a serious antagonist makes it the most probable that he is a secret twist villain.
>First appearance is setting puzzles, something that makes no sense for a "Rules Card" to do.
>Blatantly admits to want to serve tyrants
>Ch3 he reveals (at least a fragment of) his true power, can literally change how battles are done and make it impossible to attack him directly.
>Only reason we win is because Lancer neutralizes his strategy. Note that we can never actually gain an upper-hand against him in that fight.
>Can recruit darkners to his side, even after you've recruited them first.
>Cut "genocide route" boss for ch1: https://www.youtube.com/watch?v=PGF_qdUeSPs
>Appears in a beam of light like the SOUL does and nobody questions this
>Jumps into Kris' pocket even though "no one wanted that", asserting himself into other worlds (searching for power? opportunities?)
>Somehow becomes more competent over the chapters in his encounters... because he just does? He doesn't show any sign of actually learning from his mistakes. Unless he's making them on purpose?
He's poised to be a twist villain that has feigned incompetence to study the heroes and find a definitive weakness to take them down for good. It would be his style to not attack directly and wait for the perfect timing.