Hear me out- Forgot about this ask!! Sorry!!!
Cicero is my Legion oc who is a frumentarii as well. He was originally caught by the Legion as he was doing some work in Northern Arizona, and was forced into the Legion instead of being killed because he had a lot to offer- knowing multiple languages, knowing the area very well, knowing how to repair a lot of things ranging from radios-> weapons, and his strength.
He originally was with the Legion for a short while before he made a run for it back to Mexico, where he hid out for a few years. He came back for work into Nevada, unaware of how far the Legion had got and how close to NCR territory they were, and he was recognized and recaptured by the Legion.
They branded him, so he wouldn't be able to run + hide again, cut his hair, and put him through multiple methods of torture just to break him down, and he was made a frumentarii so Vulpes could keep a close eye on him and what he did in and out of camp. Cicero doesn't WANT to be in the Legion, but over time starts to accept his position and the situation, and starts to do more to attract the attention and approval of Caesar and Vulpes, giving him more purpose that isn't just "survive day to day in the mojave" but rather to become one of the more higher achieving legionaries, a realistic goal he could work toward.