In the context of the Selinaverse and Selina Mandrake - The Slayer, Athos is a vampire and a slayer who started his life as a minor nobleman French boy at the time of the First Crusade. Athos was rebellious and kept challenging the dogma of the Roman catholic priests, and as a result was eventually taught how to read the Bible by a monk. Athos read the entire Bible and later on decided to join the crusaders on their crusade. After arriving to Palestine, Athos was appaled by the destruction left by the crusaders, and ended up taking a poor local Jewish woman and her children to safety, and deciding to return to France while taking a long detour through a long trip through North Africa, and the Iberian pennisula.
After learning various languages on his way to an extent including Arabic, Hebrew, Berber, Spanish, Occitan, Catalan, and studying Judaism, Islam, Greek and Latin philosophy, etc. Athos travelled to the court of the king of France and told him that he'd like to serve France until he dies, only under the way he perceives God and Jesus. The King agreed and knighted Athos.
Athos, however didn't die and continued to serve France under various incarnations and under different identities.
Athos was considered the finest warrior in France, since he started his service as a knight, but during the time of King Louis XIII and Cardinal Richelieu, he found that D'Artagnan, who was a young and inexperienced boy from a distant village was in fact a better combat warrior than him. This proved to be a humbling experience to Athos.
Athos was eventually sent along with D'Artagnan, to escort Kate on her (succesful) mission to "slay" Lord Buckingham.
After being a Maquis fighter in France during WWII, and after the war, something told Athos that he now has better ways to fight than by serving in the French military. As a result, he asked the “King” of France at the time, wikipedia:Charles de Gaulle, for terminating his service. De Gaulle believed that Athos served France long and loyally enough and released him from service.
Since WWII, Athos has been travelling the world and universe of the Selinaverse, doing random vampire-related acts.