I just want it known that I've never actually read The Three Musketeers by Alexander Dumas, so I'm basing this solely off of the TV show. If any of the characters or hints of backstories I've given are wrong, then I apologize. Also, I'm not certain if Aramis actually is Spanish, but because he speaks fluent Spanish in the show and I've seen it hinted at in other works, I went for it.
Category: Gen
Setting: Beginning of Season 1
Characters: Athos, Porthos, Aramis, d'Artagnan, Treville.
Rating/Warnings: K+
Summary: They became inseparable, but there was a time when they were all outcasts.
Outcasts
Athos had come first. Dark, brooding, young and inexperienced but uncaring. He separated himself the moment he was commissioned—and that hadn't taken long at all, much to the other musketeers disgust. Some of them had waited years to be commissioned and here a newcomer had snatched one within a few weeks. He didn't set out to make friends, and none set out to befriend him. There was a grudging respect for the swordsman, but that was as far as it went.
Porthos came next. Set apart instantly for his background, he set out to prove himself. He practiced longer, fought harder, gave it everything he had. He fought tooth and nail for his commission and no one could deny that he had earned it. But they separated themselves from him. He was friendly, tried to make friends, but no one would approach the fresh musketeer. He wondered until the brooding man who never said a word offered to help him with his form.
Aramis came third. His presence was instantly resented and many called for the Spaniards head, regardless of how long he had been in France. A Spaniard was a Spaniard. He let the insults blow right past, ignoring them with a laugh that hid the lonely pain. He proved himself, won the commission fair and square, but that didn't stop the men from ambushing him that night. He was a good fighter, an excellent marksman, but ten to one is never fair. He was surprised as much as his attackers when two loners came to his defense.
D'Artagnan was the last. He came in with a bang, revenge on his mind and a challenge on his lips. He was thrown into the musketeer world before he was sure what was happening. But he never looked back. He was friendly with all, but attached himself to the three he had first fought. Whispers surrounded the farmer boy, and his youthful rage kept him separate from the others. He would have nothing to do with those who talk behind a person's back and he fought for the commission to prove that he was better than them.
Treville had been there from the beginning. He watched as the four were outcast, as they each earned the grudging respect and eventual admiration from the other musketeers. He had watched over them, seen the Three Inseparables formed, and he had worried about the boy who had wedged his way into their group and become the fourth. He would never say it, not even if you threatened his life, but he was proud of the four musketeers he had watched grow from lonely outcasts to the best musketeers he had ever seen.