Molly Williams is Megan and Danny's creative, generous, and very beautiful sister. She's often nervous around monsters, but never has any qualms about tagging along with Danny; she worships her brother and wants to be exactly like him, which he finds amusing and touching in roughly equal measure.
She first came to Ponyland when the ponies needed Megan's help to defeat The Smooze.
In the My Little Pony Annual 2013 comic story "The Fall of Sunset Shimmer", Molly's name (and also Megan and Danny's) appeared in a flier for the Canterlot High Sundance. She's apparently a teacher who, along with Megan and Danny, handles submissions for the Yearbook in Room E.