Optimus Prime is Megatron's archenemy and the leader of the Autobots, who oppose his tyranny and conquest.
Unicron is a cosmic entity who rebuilt Megatron into Galvatron, but also enslaved and tortured him.
Cobra Commander is a human terrorist leader who allied with Megatron, but also betrayed and clashed with him.
Serpentor is a human-android hybrid who was created from Megatron's parts and DNA, but also rebelled and fought against him.
Bumblebee is a young and brave Autobot who often foiled Megatron's plans and inspired his change of heart.