This guy is basically Dumbledore... if Dumbledore was super-athletic. He's Obi Wan Kenobi... if Obi Wan had a broken heart. He's Gandalf... if Gandalf had a completely take-no-prisoners... Wait. Gandalf does have a completely take-no-prisoners attitude.
Basically, you can think of Athos as the Gandalf of The Three Musketeers.
Also known as the Comte de la Fère, Athos is older and wiser than the other men in the group. He is extremely distinguished in just about every field—falconry, sword fighting, Latin, and bravery:
Athos and D’Artagnan, with the activity of two soldiers and the knowledge of two connoisseurs, hardly required three hours to purchase the entire equipment of the Musketeer. Besides, Athos was very easy, and a noble to his fingers’ ends. When a thing suited him he paid the price demanded, without thinking to ask for any abatement. D’Artagnan would have remonstrated at this; but Athos put his hand upon his shoulder, with a smile, and D’Artagnan understood that it was all very well for such a little Gascon gentleman as himself to drive a bargain, but not for a man who had the bearing of a prince. (38.133)
And because of the times, this distinguished attitude comes with a side order of classism:
"Gentlemen," said Athos, "my opinion is that it is not proper to allow lackeys to have anything to do in such an affair. A secret may, by chance, be betrayed by gentlemen; but it is almost always sold by lackeys." (19.137)
But don't hate Athos too much for hating on lackeys. His rigid idea of the social hierarchy is actually cast as a good thing in this novel, even though it seems pretty gross by today's standards.
But underneath all that awesomeness is a deeply tortured man. We eventually learn that a woman is the cause of his misery, and not just any woman... but an especially manipulative and treacherous woman. A young Athos fell in love with and married (dum dum dummm) Milady. Unbeknownst to Athos, Milady was already a hardened criminal. After discovering her past, he believes he hanged her (we never find out how she managed to get out of that pickle!) and went to became the Musketeer we meet in the novel.