He says he's a Darker, other characters say or strongly imply the same. It might be "less speculative", technically, to put little "or so he claims" footnotes everywhere, but at some level you have to believe what the game tells you until actively shown otherwise.

I believe he's a Darkner, even if he doesn't look like one.

Also hi mod :3

Ralsei lives in the dark world

Even if he isnt all strong and monsterish like the other darkners doesnt mean he isnt one himself

He's also a "Dark-World Being" from his title in Chapter 2. So we don't even need to trust the characters. How can you be a Dark-World being but not a Darkner?

He also uniquely vanishes when brought down to 0HP, so he clearly follows different rules than humans and monsters.

So we have more than just what the characters say. I would say we can be at least as certain that Ralsei is a Darkner as we can be that Spamton was ever really a big shot.