|Cassie and Diana|
Diana and Cassie first met after Cassie moved to Chance Harbor at school. Kind-hearted Diana was the first one to befriend her. She also was the one that tried to explain Cassie that she's a witch.
The two became friends. But their friendship was soon put to the test. As Cassie and Adam Conant, Diana's boyfriend, discovered strange connection between them, Diana started to be jealous. Adam's father, Ethan Conant, confirmed Diana suppositions. Soon after that, Diana and Adam broke up. Cassie didn't wanted to start releationship with Adam due to her friendship with Diana; she didn't wanted to hurt her.
Although their rocky history, Cassie and Diana remained friends. But last episodes, especially Fire/Ice, once more doubted their relation.
In Crystal, Cassie figured out Diana is her half-sister since Diana stated she couldn't go into the mine because she had hard time breathing Cassie knew from Royce Armstrong's informant that mine was cursed and told a shellshock Diana that they were sisters. Diana has a hard time dealing with this,because she does no want her Black magic to take her over. Like she's seen it do to Cassie so many times before.
In Family, They both found out that they are two of six Balcoin children and that John wanted them to form a circle of Balcoins. When Adam and the others are in trouble, Cassie was killing Diana to unlock her dark magic. They both used the crystal skull to kill their father. Diana wanted to leave Chance Harbor while Cassie tried to convince her to stay. She couldn't stay with Cassie while dealing with her dark magic and the two went separate paths. They both received the mark of a Balcoin on their hand.
"You know Faye's right, you really are a drama queen." Diana to Cassie in Sacrifice
"You're the circle Diana" Cassie to Diana in Crystal
"It's not that simple, we need each other, like today you barely made it to the mine in time, Adam and Melissa were almost killed in there"
"I was there in time. I heard the gunshot! I just couldn't get in...I couldn't get in, I tried, but when I stepped inside it, I felt like my lungs were going to explode. I couldn't breathe and I could barely have enough air to gag, so don't tell me what I did and didn't do."
"The mine was spelled, that's why you couldn't get in, that's why I couldn't get in...Oh my god...it's true...it's you...you're the other Blackwell in the Circle...you're my sister"
Cassie and Diana in Crystal
"Diana, you've always treated me like family and I know things are really confusing right now, but if it turns out that you are my sister, I'd be really happy about that." Cassie to Diana in Traitor "Bound or not, you are my sister. And you are the only family I have left."
"No matter where I go, that is never going to change."
-Cassie and Diana in Family
- They both dated Adam Conant and both lost their virginity to him.
- Both of their mom lost their virginity to John Blackwell resulting in their pregnancies with Cassie and Diana only 2 weeks apart.
- The earliest hint of Diana and Cassie being half-sisters was in Loner when Diana stated to the circle "It was like Cassie and I shared the same thought and it played out exactly the way we wanted".
- Faye's nickname for them in Traitor :Dark Magic Super Twin" is a reference to Super Mario Brothers.
- Even Diana and Cassie both have Dark Magic only Cassie has access it unlike Diana who has yet to do so. Since her personality is more grounded and calmer than Cassie's personality until Cassie helped her trigger the Dark Magic inside her in Family.
- Since Diana and Cassie are half sisters known as the "Dark Magic Super Twins" by Faye it suggested that Diana resemblance Yin in her relationship with Cassie. Since Yin is characterized as slow, soft, yielding, diffuse, cold, wet, and passive; and is associated with water, earth, the moon all which apply to Diana. While Yang applies to Cassie with its hard, solid, focused, hot, dry, and aggressive; and is associated with fire, sky, sun and daytime. Coincidentally those apply to their elements/powers.
- Cassie and Diana are two of six children of John Blackwell. They're both born from The Chance Harbor Circle while the others are born from the other two circles.