This is a really tough question to answer, because we don’t even know for sure that there are seven Archangels in this universe, let alone seven Knights of Hell. In Supernatural canon, until very recently (as in, the end of season seven), it was presumed that there were only four Archangels—Michael, Lucifer, Raphael and Gabriel. But then Metatron was identified as an Archangel as well, which could lead you to believe that there are at least two more unnamed Archangels out there.
In the Christian Hierarchy of Angels, Metatron was actually waaaaay way up the ranks from the Archangels. He was one of the Seraphim, the highest rank in the First Sphere. Archangels are actually only second from the bottom, the middle children of the Third Sphere. But Supernatural canon doesn’t really seem to follow this hierarchy, because Archangels are described time and again as the oldest and most powerful of the angels. So perhaps Metatron really is an Archangel in this universe.
Which, is a long way of say that this is entirely based on presumption, but if you follow this line of thinking it is possible there are seven Archangels, and therefore seven Knights of Hell. Lilith and Azazel would definitely be among them, and Abaddon obviously, but I’m a bit dubious about Alastair.
(Long-winded discussion about demon eye color and rank versus ancestry behind the cut.)
See, it had been presumed before now that demon eye color was a sort of classification based on rank. White-eyed and yellow-eyed demons were the oldest and most powerful, black-eyed demons were the most common—the soldiers, and red-eyed demons were exclusively crossroads demons. But with the introduction of Abaddon as a black-eyed demon, and the recent discovery that Crowley still has red eyes despite his very drastic change in rank, I’ve gotten quite enamored with the idea of demon eyes having more to do with the line they come from. Which is to say, these (possibly seven) Knights of Hell could each have a different eye color, and subsequently all the demons that came after them would have the same color.
Abaddon could have been the first black-eyed demon from which all the other soldier demons originated. Lilith, who was the first demon created from a human soul, would naturally have been the first white-eyed demon, which could lead you to believe that Alastair is lower in rank than she is, therefore not one of the Knights of Hell. Azazel is the only yellow-eyed demon we know of, but he is the one responsible for passing on his power to the Special Children, including Sam, and Sam is often shown in visions in season four and five as having yellow eyes.
So, we have Abaddon (black-eyed), Lilith (white-eyed), and Azazel (yellow-eyed). It’s safe to assume that one of the Knights of Hell had red eyes, which would explain Crowley and the crossroads demons. That gives us three named demons with distinct eye classifications, and one unnamed.
Henry did say that it was presumed that the Knights of Hell were wiped out by the Archangels (remember this was information dated to 1958, so he wouldn’t have known that Azazel or Lilith were most definitely still around). So it’s possible that the unnamed red-eyed Knight along with the other three unknowns were killed long ago. It’s likely we’ll never know who they were, and there are so many possible candidates that it would be really hard to nail down four other likely names.
Answering this question is really making me want to try, though. I may have to disappear into some articles on demonology for a few hours.