Unfortunately Fae would not be considered appable. I can't really give a hard number because some characters can appear in small number of episodes or scenes but have enough depth in background, personality, and canon material to work (for example, flashback characters). So it's mainly those three criteria, especially canon material, that are considered as well whether or not a lot of headcanon would be required. Appearing for only half an episode would definitely be too small. If Fae appeared for at least half of the season or had some kind of supplementary material, then it might have been more doable.

I can go ahead and answer the second question though—humanizing a character is doable and it doesn't have to be strictly human. It's possible for her to have a kind of "android" type form, and even the original black box would be an acceptable form. It would just have adjustments made so that it could move around.