Ah, I see what you mean. I wouldn't run such a thing as a Knight-type, myself- probably just a lesser Goblin Sidhe (lesser compared to the Erlking himself...).
I'm not
entirely certain why the Erlking would want a Mortal Knight, but it's a cool idea. Maybe for hunting people who "cheat" by using his Fae nature? It takes some interpretation- he was impressed when
Harry turned his own words back on him
, but I think his reaction to someone who knows he's being hunted and therefore hides in a barbed-wire wrapped iron cage would be rather different. Thus the Knight (or Huntsman, I like that title...) would be a more unconventional hunter, one who can hunt
using the modern technological world, rather than just running through it.
The person you've mentioned is relatively human for a supernatural (metaphysically speaking), so I'd say you need to be
quite human to become a Knight- human enough to choose it. If I was going to explain it, I'd say that though the Queens (and Erlking, since we're discussing him) can lend some power to their Fae servants, but there's something about the Knight package (which there's only one of per Court, according to the books) that doesn't work well with that. A Fae is what it is, and it can't change that much- so their nature can't accept the burden and rewards of becoming Summer or Winter's Knight or the Erlking's Huntsman, it'd return to being what it was after a while. A human or near-human, however, has the freedom to
choose to change themself, and by choosing to accept that power, choosing to allow "Sidhe Knight of Summer" to become part of their nature, it binds to them until their death and empowers them to do the will of their Court.