Riddle is often ending questions with だい (dai). It does not literally mean anything by itself and can be used to add emphasis, or express curiosity, or express frustration, etc., depending on the context.
Much like “dai,” かい (kai) is also a casual way to ask a question and can be used when skeptical or to seek confirmation of something, and Riddle uses it often.
Saying “dai” and “kai” so much is something Riddle has in common with Rook, who says them both just all the time.
Riddle is also often saying よろしい (yoroshii), close to an English equivalent of “acceptable” or “satisfactory” or “okay.”
A line that Riddle also says before battles, he will often say “ii dokyo ga oari dane,” which is something like “to have some nerve.”
Other characters will say variations on “ii dokyo” (usually Vil and Leona), but this full phrase is unique to Riddle.
Riddle is also often saying おわかりだね (o-wakari dane), which means close to “You understand?”
He will often use it when threatening people with a beheading (similar to “If you don’t do as I say you know what will happen to you, yes?”) but also in lower stakes conversations as well.
Twisted Wonderland Reference Note 