In descr_strat in the campaign folder (data\world\maps\campaign) in the appropriate campaign, search for Flavius, and it will come up with his entry.

This is a list, at the top it says 'character' and goes on to list his attributes and underneath it lists his army.

In the list are his traits like GoodCommander 2, taken from export_descr_character_traits.

Add as many as you like to the list. Good luck.