Athos

Olivier d'Athos (mostly referred to as Athos) (born 1600) is one of the four titular protagonists of D'Artagnan and Her Musketeers

History/Role in the fanfiction
Milady reveals herself to Athos and prepares to kill him. However, Cosette rescues Athos from the burning manor. Upset and delirious, Athos reveals to Cosette why he acted so strangely: it's because he felt his wife's presence and tells Cosette that he had his wife executed when he learned she was a criminal. This conversation also acts as a catalyst for Cosette's guilt of hiding her gender from her friends.

Cosette D'Artaganan
Initially unaware she was a woman, Athos genuinely sees d'Artagnan as a younger sister. He was initally furious at her for keeping her gender hidden and refused to speak with her.However, Athos forgave her when Cosette could have been killed by the madman holding her, Constance, and the king hostage. He realized that he nearly lost her. Despite his protectiveness over her, Athos still allows her to come with them on missions and lets her do things that original d'Artagnan would have done.

Athos is also the only one who calls Cosette "Cos," a nickname her father used to call her and a nickname Athos used to comfort her when she was sick.

Trivia

 * Athos was the last Musketeer of the Inséparables to know Cosette is a woman, being preceded by Porthos and Aramis.