The Cybertronian who would become Optimus Prime was once a simple data clerk, satisfied with his job. It wasn't until he heard of an outspoken gladiator named Megatron and his call for an upheaval of Cybertron's society that he realized he was dissatisfied with his lot in life- and that the oppressive caste system hindered the freedom of all the sentient beings on Cybertron. Little did he realize that his boss, the archivist Alpha Trion, knew of a secret potential within the data clerk, and that Cybertron's history was at a precipice. Once he and Megatron discovered they did not have as much in common as they once believed, the lowly data clerk was promoted by the High Council to be the next Prime.
Over the centuries, loss and war have led Optimus to appear stern and serious to most. But scratch the surface, and you'll find an individual who greatly cares for his fellow living beings, with a warmth and kindness his closest friends can rely on. You'll also find regret over the loss of a comrade to darkness, and the faintest glimmer of hope that he can be saved.