Saras are yellow and take on an orb shape that looks slightly like Shannons head. They have one eye, similar to Ernestos and Darrells, and a red siren attached to the tops of their heads. They don't have any physical bodies.
She will pop out of the ground and can be picked up and thrown at enemies. Like a bomb, she will explode when detonated.
Sara's name, along with her brother Teagan, is a reference to the Canadian band Tegan and Sara.
Sara and her younger brother Teagan have not shown up in the show yet, so there is a chance that Sara and Teagan aren't actually canon, at least in the show's universe.