The Autobots' leader is the biggest, strongest, and smartest. Optimus Prime makes the plans and protects the humans in the war with the Decepticons. You wouldn't call Optimus Prime cuddly, but he does care. He's just seen a lot of battles ... and he knows there are more to come.
This giant is the muscle of the Autobots. But Bulkhead's actually sensitive - as Miko learns when she makes friends with him. When Miko's in danger, Bulkhead's there to protect her.
He's the Autobot who's been on Earth the longest. So he knows humans best. Bumblebee's voicebox was damaged on Cybertron, so he "talks" in bleeps and blurts.
This emergency medical truck is the Autobots' medic. Ratchet fixes limbs and gears, and he doesn't want the Autobots risking their necks to protect humans, or bringing them into the secret base. Deep down, though, Ratchet admires what the humans bring to the fight.
The head of the Decepticons is just as bad as the Autobots' leader is good. Whether in alien fighter jet form or robot mode, Megatron's the most evil and deadly of all the Transformers. He hates how much Optimus Prime cares about Earth, and won't stop until he defeats the Autobots and makes this planet his own.