A female character helping the Legion would get sold as a slave in the end, or nailed to a cross. The legion is just not capable of dealing with such an advanced idea as a woman with power.
Doubtful Caesar isn't stupid. Even if you did every little quest to disrupt the legions plans before heading into the tops to confront Benny. Caesar still offers safe passage To the Fort and a pardon.t Why? because he knows your extremely powerful and in his own words when you set your mind to something it gets done. He wouldn't foolishly throw away such a valuable asset.
Women aren't allowed to be legionnaires and the women of conquered territories and certainly incorporated tribes may be enslaved. But not every woman they meet gets the slave collar put on them. There is actually a female-only caste of priestesses who maintain the cult of Caesar among the slaves and members of the Legion and also take the children of slaves to be raised on Caesar's doctrine.
And really who would want a slave fully capable of taking down an army who probably has the technical skill to remove the slave collar at any given time. From a historical sense, If Joan of Arc could become a military leader in 15th century France. A woman could become a military leader in the Legion.