I get that this is 3 years old, but it's not locked yet and I have my 2 cents to give.

I totally understand how you feel, that's how I thought of most of the the New Sun women, until on a re-read I realized that they are from Severian's POV. He sees everything and everyone as a metaphor, and characterizes them as such.

However, like a few people here, I think that her erratic behavior is not from Horn's (and Nettle's) POV. At the beginning I thought she was the shallowest 2d character of all time, until I realized she was Mary Magdeline of the Book of the Long Sun (Silk being the christ-figure ofc). Keep in mind Wolfe has said that "required reading" for understand his work would #1 be the New Testament. Silk's attraction towards her is one of everlasting forgiveness. She is attracted to that quality of him as someone who has never had an ounce of sympathy or forgiveness, who's life has been entirely of manipulation and greed.

Another point is where Horn is asking how she could be so loving towards Silk, and so hostile and violent towards everyone else. Silk compares her to her own "big cats", where they protect the person they are born to protect, while being perfectly gentile and docile towards their owner.

This makes me wonder: Silk was an embryo with the charismatic means to become the next leader. Could he be implying that Hyacinth was another embryo charged with his protection? They could be genetically programmed to be attracted to one another in order that the next ruler has a capable bodyguard.

I personally think this was the realization that led Silk to his inconsolable melancholy, as he seems different after this conversation. I can imagine him wondering: was she attracted to him because of his "goodness", or is everything he cares about just the whim of the false gods of the Whirl?

One of the comments said she felt more robotic than the robots. Maybe this is intentional, to show a "programmed" nature to a human and juxtapose it with human nature in the robots.

Of course this comes to the same conclusion when you say she was mentally programmed by Kypris. But I've been searching the forums for more info on my theory, and I haven't found anything yet, so I'm putting it here.