Characteristics & Looks of a Roman Soldier

At least in later periods, Roman soldiers had a tattoo on their hand/arm to mark them as a member of the army. Many believe this was intended to deter desertion. However, this may not fit the period your story is set in.

 

His accent would far outlast his chin strap mark I would think.

 

If the man is an ethnic Italian Roman, he will be shorter than most other ethnic groups. Julius Caesar was considered tall for a Roman at a towering 5'6". I have no idea how tall a Scythian or Sarmatian would be, but I know from many sources that Romans were much smaller than almost every group they encountered. This may be a bit more complicated, as the Romans spread and intermarried with everyone they conquered, and those people were considered as Roman as native Italians after a few generations.
As far as the steppe tribes are concerned, there was a great variety in physical characteristics. Dan Carlin talks about this in several of his podcasts, as the steppes were an area of great migration and genetic mixing, so it would not be unusual for a Scythian, Mongol, Turk, Hun, or whatever to have any number of ethnic traits. It would not be unusual for someone to have dark, middle eastern features with blue/green eyes or blonde/red hair. The point is that physical appearance won't be any particular giveaway.

As was mentioned, the Romans did tattoo their legionaries to an extent, but this was mostly for foreign auxiliaries, not native Romans. Criminals and slaves would be tattooed or branded, and most Romans would not want to associate themselves with that social class. Steppe peoples marked themselves regularly, making them that much more bizarre to the Romans.

Other than the movie "The Eagle" I have heard no mention of a chin strap causing permanent scaring to a legionary. I'm not saying it didn't happen, I've just never heard of it. Really, there is no physical characteristic that would give him away, so you will have to rely on his behavior.

 

Along the track that Russ was following....perhaps rather than physical markers, behavioral markers would be better clues. Maybe in the way he holds a spear, or rides a horse, or eats his food....I don't know what, precisely, will work best. But cultural influences on habits could be long lasting, hard to erase.

 

At that time, tattoos weren't much of a thing. They settled in in the 200s and 300s AD. However, for the time you're set, a Roman soldier would definitely have an Italian look about them.