Rex

Rex was a veteran clone trooper captain during the Clone Wars, but he did not participate in Order  66, as he had his chip removed. He was usually Anakin's first in command.
He appears in The Mortis Trilogy, which he tells Anakin, Adyson, Ahsoka and Obi-Wan they are at the rendezvous point. He appears in the third part, tellign them they lost them on the records for a second, but Anakin says they've been gone for more than a moment, but both realize it must have no effect on them. 
In Mysteries at the Temple, Rex appears, refusing to believe that Ahsoka is responsible for the death of many. However, he reluctantly has an IO called on her for being "armed and dangerous".