I too like the maturing friendship between Gil and Tarvek. If I remember correctly, they were childhood friends who met when they were both on the airship city, when Gil was incognito among the other students/hostages, became estranged, then met again when both were students in Paris, where the impulse to be friends was still there but each thought the other was a fool. Heh, they were spark teenagers. Now they are coming to appreciate one another as adults.
And I found it interesting that when Zeetha turns up in the story the first thing Klaus assumes is that she is there to kill either himself, Gil or both. Whereas from Zeetha’s own point of view she is simply lost. Klaus assuming that makes my guess that he is her father seem more likely.