Not even thinking about the mech, if I wanted to go for maximum drama, I'm thinking of a mercenary who used to be in both the Imperial Vanguard and the Royal Guard. Starting as just a soldier in the Empire, he rose through the ranks and became inducted into the Imperial Vanguard. As his first assignment, since he hadn't become known to the Kingdom yet, they had him go into deep cover and infiltrate the military of the Kingdom. Rising through the ranks again, he became one of the Royal Guard, and served as a knight. His assignment was to see if he could assassinate the Queen, or at least feed critical military information to the Empire that would allow them to crush the Kingdom. However, time spent in service of the Royal Guard compromised him, and he was unsure of what he should do. Of course at this point, the Empire found out he was wavering, and so when he tried to meet his factor one night, they tried to kill him. He survived, but not without the Royal Guard finding out who he was. So a traitor to both sides, he had to run.
Officially, he doesn't exist in any military's history, because he's a deep shame for both sides. But both elite units remember him and assassins from the Empire regularly try to hunt him down and kill him. Meanwhile, he takes odd jobs, and even the Kingdom has been known to hire him, to the incredible distaste of any of the Royal Guard who know his past.