CT-27-5555, called Fives, is introduced as a rookie on the Rishi moon in The Clone Wars' first-season episode "Rookies" (2008) and becomes an ARC trooper in the 501st. He is voiced by Dee Bradley Baker.
His arc is the series' spine, and it is a detective story. After the clone Tup malfunctions and kills a Jedi, Fives investigates, finds the inhibitor chip in Tup's brain, has his own removed, and works out that every clone in the army is carrying a device designed to make them obey a single command.
He is right, and it destroys him. Drugged, discredited, hunted through Coruscant by his own side and shot by Commander Fox, he dies in Rex's arms sounding like a paranoid. Six seasons later Rex remembers the conversation, and that memory is why Ahsoka Tano survives Order 66.
"Fives" is the most complete piece of tragedy in the animated canon: a soldier who discovers the plot, tells the truth, and is not believed until everyone he warned is dead.