In The Three Musketeers, he and the other two musketeersPorthos and Aramis are friends of the novel's protagonist, d'Artagnan. He has a mysterious past connecting him with the villain of the novel, Milady de Winter. In one part of the book, he drunkenly implicates himself as the murderer of a sixteen year old girl, which disturbs d'Artagnan.
The oldest by some years, Athos is described as noble and handsome but also very secretive, drowning his secret sorrows in drink. He is very protective of d'Artagnan, the youngest, whom he treats as a son.
By the end of the novel, it is revealed that he is the Count de la Fère, who was Milady's husband before she married the Baron de Winter.
In the latter two novels, he is openly known as the Count de la Fère and is the adopted father of the young hero, Raoul, vicomte de Bragelonne (and it is ultimately revealed that he is his biological father as well). In Twenty Years After it is left uncertain whether Athos is the father of Mordaunt (John Francis de Winter, son of Milady de Winter). Mordaunt attempts to avenge the death of his mother by killing those responsible. However, he himself dies while he and Athos struggle in the middle of the English Channel. Athos himself almost drowns in this struggle as well causing Aramis, Porthos, and D'Artagnan to lament his death for a few moments before he resurfaces.
Athos' first name is never told in the novels. However, in Dumas' play "The Youth of the Musketeers," the young Milady, then named Charlotte, calls the then Vicomte de la Fère, Olivier.