Using console commands on individual characters isn't easy anyway, and the Romans use a different naming system then the rest.

Here is a thread about using the move_character command. You can use the instructions to identify the character's internal name.

After that you need to know what the console name is for the trait you're looking to add. I think it's in exprot_descr_character_traits or something like that. I recall it was "high ambitions" in the game, but for the purpose of console commands the internal name might be different.

In the end the console command would look something like this:

give_trait: "name" high_ambitions 1

The 1 you add signifies that you're adding the level 1 version of the trait - some traits have multiple levels, like brave -> very brave.