SB

SB is short for "Sleeping Beauty". The original Emma Crewe is kept trapped in a sleeping, dreamless state while her Changeling self, Emma Crewe the Skeleton Key, walks around free. It took years before the truth was discovered. Thanks to Adora, both Emmas were freed from each other, but SB's identity and freedom remains problematic.

Childhood:
SB was the original Emma Crewe in all ways. She says her mother sold her to One so he would have a Skeleton Key fed full of stories by her father, Daniel Crewe. Daniel missed his daughter terribly, and he grew to resent his role in feeding stories to his changeling daughter. He quietly began to rebel, letting her hair knot up (a sign of a changeling), and when he could, he used the last of his Writer powers to drive her away to protect Elaine, whom they both loved.

SB has spent most of her life in a medically-induced coma-like state. We see One and Two (Ina Liddell) guarding her, for the link between the two Emmas was important for the Skeleton Key's growth.

Briar
One named SB as Briar to gain full control over her. He openly treated her like a slave and when she rebelled and refused the name, he seemed contemptuously amused more than anything else. Knowing One, he has something perfectly awful in mind for her. We know one of the vials of energy he had her drink before the Council of Nine meeting was...nasty. He calls it, 'A Lindworm's Tale'.

SB is much like the other Emma. She is brave and has no patience for bullies. She grew up with Emma mentally, a powerless voice only able to watch her grow up as herself. Only with Emma's rising powers as a Key has SB been able to collect enough of her own energy to meet her 'twin' in dreams and counsel her on future moves.

We'll be keeping an eye on SB.