Ambrosius Aurelianus
We know from Gildas that he was of high birth, and had Roman ancestry; he was presumably a Romano-Briton, rather than a Roman from elsewhere in the empire, though it is impossible to be sure.  It also appears that Ambrosius was a Christian: Gildas says that he won his battles "with God's help". According to Gildas, Ambrosius organised the survivors into an armed force and achieved the first military victory over the Saxon invaders. However, this victory was not decisive: "Sometimes the Saxons and sometimes the citizens (meaning the Romano-British inhabitants) were victorious."