Kalin of High Rock
Faal Lun Vahdin
Actually, it's not subjective at all. The Greybeards fully accept you as a member and yet have no intention of telling you that Paarth even exists. That's deception and it's not admirable.
I'm not sure you're remembering it correctly.
The first time his name comes up (that I can recall) is after you've mastered Whirlwind Sprint shout. The dialog option comes up "There are only four of you?".
The Greybeards make mention of him as the leader of their order. Which is entirely true. Their language may be misleading; as one would naturally assume that the "leader" of the Greybeards is himself a Greybeard. They say he lives on the peak, which is also true, and elude to an eventual meeting when the Dragonborn is deemed "ready" (Whatever that may mean).
They never mention that he's a dragon. But in his capacity as their leader that fact is irrelevant.
That's not disclosure, alluding to some phantom leader. That's deception no matter how you want to paint it. Sorry. At the very least they should have introduced you to him after retrieving the horn. The fact that he is a dragon is not irrelevant at all, in fact quite the opposite. Dragons are reappearing and the fact that he is a dragon is hugely substantial. Obviously since the quest halts without their participation.
You're kind of backtracking a little bit, which makes having a conversation difficult to enjoy.
Before you said they made no mention mention of him. At all. Having "No intention of telling you that Paarth even exists". When that's proven to be false, you come back and say that they didn't "disclose" enough of his existence for your liking.
They tell you everything about Paarth that's relevant to The Dragonborn's line of question. His name, location and his role as leader of their order. The fact that he's very wise and seldom seen. Hardly a phantom figure.
What they don't do is volunteer unsolicited answers about his race. While you may find the fact that he's a dragon relevant in the context of events unfolding outside of the monastery, *and it may be. The fact that he's a dragon is irrelevant to the questions being asked of them.
The Greybeards are never flatly deceptive.