I called it in the first chapter, but his chapter drops a more concrete hint: Ruri almost certainly picked-up a different job from the book at the same time Noah became a Monster Tamer ― most likely, the Hero job. Unless, there actually is a Demon Lord job and she's that, instead. The story so far seems to insinuate that Lilcarla, the previous "Demon Lord," was just a Monster Tamer, like Noah. So, there's basically two ways I see this playing out: 1) Monster Tamers are considered Demon Lords / there is no actual Demon Lord job; 2) There is a Demon Lord job, but Noah isn't it / Ruri is the Demon Lord, instead. It could also be that neither of them are the Demon Lord and they team up to beat the actual Demon Lord in the end to prove that Monster Tamers aren't evil, though I think that sort of ending would be far too cliché.