Your canon material for study: 'That Naughty Flashback Scene'
Ready, steady, GO!
(Nod) I agree he seemed quite fixated in the present. I have three points that keep me from being at all sure what his original interest was.
He cites his position with the Knights first, and seeing her only second. Oh -- and the lack of indication why he wanted to see her. "I've been waiting to tell her just what I think of her for twenty years," seems possible still.
Vrin's conviction he wasn't loyal to Lucretia/The Goddess, but merely wanted to make use of her.
Last,and for me both creepiest and disturbing, is that Lu was using mind control. We had known about the wasps, and we'd known about the downloading technology of the Summoning Engines. But there are those three tubes down in the Lair. I find it beyond believable that she had tubes and wasn't using them... especially when she is able to comment on the comparative advantages between wasps and mind control tubes. We also know just from her comments that the tubes work just fine on Sparks: she's planning on using them on Gil and Tarvek. Zola is at least expressing a similar type of willingness to tamper when, in her frenzy, she announces she's going to have Tarvek brain-drilled to love her. I am... uneasy... that some of Lu's suitors were unwilling. Klaus seems like the least likely to have been meddled with...if only because he remains "so over her!" Aaronev, with his entirely out-of=proportion behavior, seems the most likely. And, with the novelization coming out, and signs that Bill was going mad after The Fall of the Castle, seems at least possible.
I do think Aaronev looks like a good victim of mind control. He would have to be incredibly, utterly insane with love otherwise. It's one thing to ally with the Mongfish. But he appears to have then specifically handed control of his ancestral orders to Lu, he allowed his own offspring to be meddled with, and he remained blindingly obsessed with her 20 years after her disappearance. I have plenty of other theories -- some which may play well with mind control, some of which may prove to exclude the possibility. But mind control continues to look plausible.
The point about Lu using mind control triggers the English teacher in the back of my head-- if this triangle echoes that of the last generation, and they're not going for pure cycle, then they're probably setting up a compare/contrast with the modern being an improvement.
It looks like Lu/Ag might be Love Woman, with the older corrupted by power. (Think kinda like the Mother version-- Archtypical Terrible Mother controls and destroys her children or feeds on them, the good one supports and gives but lets go; Clytemnestra vs Virgin Mary.)
Gil pretty much HAS to be this generation's version of Barry, I'll call them the Idealist, but where Barry is blind, Gil has more savvy.
I think that would put Tarv as the Realist (for lack of a better term), and if we can figure him out, we'll be able to figure out Klaus. My gut says that Klaus's flaw is being bitter, over-generalizing in an effort to be realistic. (The flip side of idealistic.)
Depending on how deep they end up going, I may end up having to get a spare copy of all the books and have my kids write English papers on them....