Roller

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Roller is a small, cart-shaped device that Optimus Prime uses to slip unobtrusively behind enemy lines. He can maintain radio control over Roller at distances up to 1200 miles. Having Roller present somewhere is like being there himself. Roller is a droid, able to function autonomously without Optimus's active oversight. But without Optimus, by far the most important of the three modules, Roller would not survive. Optimus shows great attachment and concern for Roller, who is left particularly vulnerable by the behind-the-scenes situations in which it specializes.