Ambrosius was a war leader of Roman-British decent. Surprisingly the earliest mention of him is in the 6th Century by Gildas. Gildas named very few people in his sermon De Excidio et Conquestu Britanniae, but he does name Ambrosius. This is what he had to say...
Ambrosius was of Roman descent.
Ambrosius fought the Saxons.
"Sometimes the Saxons and sometimes the citizens were victorious."
Sometimes it is argued that Ambrosius is Arthur. Both fought at Badon Hill, both were powerful warlords.