Feather --> Eagle --> Bird --> Sky --> Olam Hével? Okay, but what if we didn't know his old nickname?

His Skype is listed as nebo_24 in his profile. The problem here is different: when creating a clue, the GM is working from the player's profile to the final clue, and not the other way around. That is, the sequence should not be "Feather ---> bird ---> sky," but rather, "Sky ---> bird ---> feather." This doesn't look very natural.

Added 16 minutes later
I'm not insisting on the correctness of my guess (moreover, I'm almost certain it's wrong). I also can't connect the second clue to the Ghost. But in terms of behavior, he is much more like a Negative – he has that characteristic caution, as if he doesn't want to say too much... and the fact that the clues don't fit well together could mean that he's not a professor, but a military recruiter or a potato farmer. It's hard to say anything definitive about Nebko, but the risk of being wrong with him is much greater. And executing Mirka in the current situation is EXTREMELY undesirable. Therefore, I'm keeping my vote on the Ghost until a more worthy candidate appears. You can do as you like, but I wouldn't advise spreading votes around.

Added 2 minutes later
Ghost and Nebo, you could at least write something too.