Harkiva

Originally one of the "always three" with Dakaria and Emma, Harkiva was very close to Dakaria and "the best of us".
Having discovered the negative effects of human blood, after being forced by tradition to drink her first drop and then studying the adults thereafter, Harkiva is the reason why the pact swore off of it.

Even being "from an aristocratic family," she beared too much greed and ambition for someone who didn't know how to stop. So after the Circle found her, she changed into someone else and left the pact. Possibly due to greed and possibly for her family's protection. Either way, her actions made her the trampler of the seed of salvation she planted herself. And now, she's not only one of the most powerful blood sorceresses Emma's ever seen. She's also the shadow, protegée, and "spoiled child of Izalith."

Forgiven by Dakaria, but not by Emma, Harkiva's betrayal (or abandonment of the three-person pact) seems to have caused the closest thing to a split between the two.