Schierke is shown to have some sort of affection for Guts, much like how Flora shows affection towards Skull Knight. Zodd seems to be able to go toe-to-toe only with Griffith, Guts and Skull Knight. In that sense Gaeiseric probably had more in common with Ganishka, trying to rule the world and rebel against God. Entirely possible feelings developed, maybe one sided ones. Skull Knight isn't all knowing so he had to make an effort to know these things. Skull Knight telling Guts that restoring Casca's sanity might not be what she actually wants. Skull Knight is most probably Gaeiseric and Gaeiseric had a large kingdom comparable to Genghis Khan or Alexander the Great. My theory is that SK is simply making Guts his apprentice the same way Flora did Schierke. level 2 Flora simply had a much easier time or so we are lead to believe. If Guts and Schierke are any indication, Flora was a member of whatever party/quest Skull Knight had. We have to assume he killed an enormous amount of people to reach that goal.