Athos and D'Artagnan

This is the relationship between best friends and fellow Musketeers D'Artagnan and Athos.

Overview
Though best friends through most of the series, Athos and D'Artagnan were initially bitter enemies due to the latter believed the former to be responsible for his father's murder and even challenged him to the duel. Throughout the fight, Athos had done his best not to harm d'Artagnan with their swordfight. However, the two men are both on the sword-fighting level. Later, when Athos was leaving the execution area, the two of them just briefly nod, looking at each other as Athos silently thanked d'Artagnan.

By episode 2, Athos felt responsible for the young man, and did not want him to get himself killed in exposing Valdim and infultrate his den of thieves. However, after seeing D'Artagnan face Vadim, he accepted D'Artagnan could take care of himself.

In episode 3, Athos finally confesses to D'Artagnan his darkest secret, and promised to Athos he would not tell anyone else. During "The Challenge," Athos advised him how to fight with his head and not with his heart and expressed that d'Artagnan was ready to get a commission for the king.

"Friends and Enemies"
D'Artagnan and Athos meet each other in "Friends and Enemies, where the former thinks that the latter had killed his father, Alexandre D'Artagnan, As pointed out by Porthos, during their duel, Athos fights in  a way that ensures he won't harm D'Artagnan. After overpowering D'Artagnan and pinning him down to a nearby stable, Athos spares the young man's life. Blinded by anger, D'Artagnan refuses to let go without a fight and continues to fight the man that would have killed him.

Aramis and Porthos soon join in but Athos doesn't want to kill D'Artagnan and ceases the fight when Constance enters the garrison. When he is arrested, Athos tells D'Artagnan he is not responsible for his father's death. D'Artagnan spends the entire episode helping prove Athos' innocence despite nearly killing him a few hours earlier. He does help prove that Athos never committed the awful crimes he was accused with.Despite their previous encounter, Athos indirectly thanks D'Artagnan for helping save his life.

"Musketeers Don't Die Easily"
Athos learns that d'Artagnan has slept with his wife and shot d'Artagnan while he was drunk as part of their plan but instead had shot him in the side rather than in the arm. Apparently betrayal, Athos and d’Artagnan are actually putting on an act for Milady and the Cardinal in order to finally bring justice upon them. Genuinely, Athos apologizes for giving D'Artagnan the wound, never meaning to actually give him that much bodily harm. Despite having to "kill" Athos, d'Artagnan instead gave him a genuine smile and put on a convincing act in shooting his best friend. Later in the episode, while d'Artagnan is confronting the Cardinal, Athos primarily is concerned with his younger friend’s well-being, so much, that Athos wants Constance’s disappearance to be kept secret from D'Artagnan in order for this stunt of his to not cost the young man's his own life and to prevent a lapse in concentration.

Series 3
The series finale reveals that Athos decided to pass down his title of captain to d'Artagnan for a temporary leave of absense.