Sadie is a holographic human driver Arcee projects in order to remain disguised from the humans anywhere she goes.
Arcee activated Sadie while moving around Jasper and later turned her off shortly before Jack noticed her.
Sadie was active by Arcee during their clash with MECH on the road before being turned off when the Autobots were forced to transform to battle a small group of aerial Vehicons.
When Sierra saw Sadie on Jack's motorcycle (Arcee), Jack lied that it was his mother driving it. Sierra didn't appear to believe him and commented on Sadie's attire before leaving.