I know everyone’s thinking Faye at the moment (the assumption being that Cassie and Faye might be half sisters, if Dawn really did have more than a crush on John as was hinted at during the fundraiser between Dawn, Charles, and Jane), but that seems too obvious.
Plus, it might be worth noting that Melissa is the only one in the Circle who is biracial. I thought this was purely a casting choice since they were condensing the series for television and instead chose to loosely base an “original character” off multiple characters from the book, so as long as they were doing the “original character” angle, they might as well add in some skin pigment diversity too. (Don’t even get me started on how irritating it is that so many shows are way too “white” - only albinos are truly white - skin dominated and then when they do introduce non-“white” characters, they almost always get the short end of the stick. I was very happy that they went that direction with Melissa, even though I wasn’t sure what kind of intentions they had when they did it.)
But now with the whole Blackwell revelation, I’m wondering if maybe her biracial lineage might also be a plot point? Instead of the Cassie and Melissa half sister angle, could one of Melissa’s parents be an “illegitimate” Blackwell/Balcoin descendant?