User blog comment:B214/Question and Answers 2/@comment-25222726-20140609153437/@comment-16281343-20140609154535

But ofc, there's the pure-blooded servants. (not reincarnated ones but real devils) :D

So, the safe way to say it is " pure-blooded Devils in prestigious houses are born High-class Devils "

~Imo, making contracts can take you as far as High-class devil but not Ultimate-class. You need to have some accomplishments under your belt and be acknowledged by the Underworld.