For a start, even though he was keen on being friends with Eve, he was still determined to help Katherine find Project Eternity and didn't care about using Eve.
After he kissed Eve, he still said to Katherine that he could still get Eve back on side, finish the job, therefore carry on spying for Katherine.
When Katherine gave him another job to do in series 1 episode 8, instead of saying something like, "I'll try", or, "I think I can have a go", he so-very-confidently said "I can do that", like he was trying to impress her. And when she said to him "Zac... I'm depending on you.", he said in a really loyal way, "I'm on it.".
Later on in the same episode, when Katherine asks where he's been, he says "Actually, Katherine, what I do in my own time, is my business." in a tone that implied that he was doing something with Eve and he was deliberately trying to wind her up.
Plus, he called her Katherine, even though earlier on in the episode, he called her Dr Calvin which implied that he respected and admired her. But then later on, he got all cocky and called her Katherine.
Then even later on in the episode when he found out about Eve, he didn't care about telling Katherine about Eve; she'd be happy with him for telling her.
He even said to Katherine "I was completely taken in, I can't believe I nearly-" as if he was trying to make Eve look like the bad one.
Plus, when he touched Katherine's arms like that, and she pushed him away, his face says it all, and he quickly carries on talking, as if he didn't just inappropriately touch her.
When Will was trying to convince him to keep quiet about Eve, he was very nearly persuaded, and then Katherine said "Zac, get in here, now!" And he instantly changed his mind and went to tell her, but Will stopped him.
Then when Will said, "... And be Katherine's lapdog, for the rest of your life", he decided to keep quiet. What if he decided to leave when he realised that Katherine would never see him in the way he saw her?
Then in series 2, I don't think Zac quite dropped his crush on her (assuming he does), because when they were trying to track down Eve, Adam and Mary, even though Lily was already doing it, Zac was like, "I'm on it!" going to the lab computer to help. What if he was trying to impress Katherine again, and get back on her good side?
Side note:
At the end of that episode (series 2, episode 8), Eve goes to Will and Zac and says "Will, we're leaving now.... Goodbye Zac." Well Zac didn't make any indication to leave, and he was still there when they were trying to save Adam (who'd been hit by an EMP).
So after that, what if Zac... I don't know, made a pass at her?
Before you say anything, I do have a theory behind it.
In the next episode (Giving Up The Ghost), he hung out with the gang, didn't go anywhere near Calimov.
Then, in series 3, episode 4, when he comes back and he's in the private lab with Eve and KT, Katherine turns up and his facial expression looks a little awkward when he sees her. He looks away quickly and fidgets. Then when he says to Eve, "Eve, wait!" because he doesn't want Eve to leave him alone with Katherine, since he's avoiding her? But then Katherine stops him and says "Not so fast... I want to have a chat about the information you've been giving Nick." But her tone implies that she wants to talk about something entirely different, like she's hinting (like him trying to kiss her?!?) Plus, what the fuck is she talking about, "Information he's been giving Nick"??! It doesn't make sense! When was this, then??
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I don't know if you guys agree, but I'd love it if you gave me your opinion on this. I actually came up with the theory when I was writing Dear Future Husband yesterday.
