Britt Robertson, Shelley Hennig

The other Balcoin child on The Secret Circle has finally been revealed!

Upon learning that John Blackwell (Joe Lando) had fathered another witch in the Circle, the odds on it being Faye (Phoebe Tonkin) were favorable, especially considering her mother's crush on Blackwell and Faye's own affinity for the darker side of magic. But it was innocent Diana (Shelley Hennig) who turned out to be Cassie's (Britt Robertson) half-sister. Twist!

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Now that Diana knows she's got dark magic in her, will she also go down the same slippery-sloped path as Cassie? TVGuide.com caught up with Hennig to get the scoop on how Diana will handle her newfound family, and how Charles (Gale Harold) will feel about the discovery that his now-deceased wife was unfaithful.

How did you feel when you learned that you were the other Balcoin child?
Shelley Hennig:
I was shocked, but also was very grateful that they chose Diana. I thought it was a cool choice especially because it explains Diana's connection with Cassie that she's had since Episode 1 — why Cassie would be the first one she'd go to after she and Adam (Thomas Dekker) broke up, when Cassie was the one who was the reason they broke up! I had no idea which way they going to go. It could have been a guy. It could've been Adam, which would have been weird.

Did you have to keep the reveal a secret from the rest of the actors?
Hennig:
No! They knew before I did. [Laughs] They were walking around talking about it in front of me and I overheard and I was like, "Wait, what? Why didn't you guys tell me?" And they were like, "We thought you knew!" I didn't believe it until I was actually shooting it anyway.

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How will Cassie and Diana deal with the new information that they're half-sisters?
Hennig:
There's definitely going to be a new dynamic, obviously, but maybe not the greatest one. It also means that Charles isn't her father. So, more importantly, she's thinking about that and thinking about how everything she's ever known is not true. How do you deal with that? I don't think that she's excited about it, that's for sure.

Will she reveal the news to Charles?
Hennig:
She's going to confront her father for sure because I don't think she believes it initially. She needs more conformation, so she confronts him and they both just keep finding out more information.

What will we see for Diana and Blackwell?
Hennig:
Charles warned Diana that Blackwell is not a good guy and to stay away from him. Cassie's developed a relationship with him, Jake's (Chris Zylka) developed a relationship with him, so one person against five others, you can't just come in and say, "You can't do this!" Especially because Diana is not really the leader of the Circle anymore, so she has to strategically let these guys know that Blackwell is not to be trusted. With that said, he's also the only answer to finding the crystal, so that's the obstacle for Diana — not trusting John Blackwell, knowing he's bad and still having to use him to get what they need.

Now that Diana knows she has dark magic in her, will she play with it? Or will Cassie's experience warn against it?
Hennig:
The last thing Diana wants is to use her dark magic. It's so foreign to her. But Cassie needs Diana in order to get more information and do her spells. That's the only way for them to save the rest of the Circle and for us to survive. Diana has to tap into it.

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How will the rest of the Circle, especially Faye, react to the news that Diana is another Balcoin child?
Hennig:
Faye is jealous. She's totally jealous, she wanted it to be her for many reasons. She never knew her father really, so what better father to have than a guy with dark magic?

Now that there are two Balcoin children within the Circle, how will they fare against the Hunters?
Hennig:
Now that Cassie's explained to Diana that we need her, that puts Diana in a power position. She can use that to her advantage and say, "But we're going to do it my way. We're not going to trust John Blackwell even if we have to, we're not thinking that he's our savior." Diana comes into the leader position again.

What can you tell us about the finale?
Hennig:
You'll see Blackwell, Diana and Cassie team up in order to save the rest of the Circle, or one person in particular maybe — with that comes a spell that they have to do using their dark magic and it's just a really cool spell that causes even more peril. It's just a matter of Diana hoping Cassie's not falling back into John Blackwell's arms, so Diana is completely on edge. There's a moment where it looks like Cassie's falling back into thinking John Blackwell's the good guy, maybe it's the power taking her over, but then you realize it's all about survival for Cassie and Diana.

The Secret Circle airs Thursdays at 9/8c on The CW.