"Arrogant bastard, aren't you?" had been the first thing Aramis had ever said to him, and Porthos had shrugged and grinned in a way that showed more teeth than sincerity. If he hadn't walked into the garrison like he had the right to be there, someone would have failed to notice his uniform and told him the stables were around the back. Porthos had, incidentally, punched Aramis in the face not long after, though that had needed additional provocation.
The first thing Athos had said was, "Let us see if our friend here has any substance to his swagger." That had actually been an invitation to try to punch him in the face, which shortened the whole affair considerably. Though it wasn't swagger to keep your shoulders back and your hands loose and near your belt, nor to walk like a fighting cock and look everyone right in the eye; it was survival.
"I like the look of you, young man," Treville had told him, shortly before Porthos had met the others, and long before he'd (mostly) stopped coming to blows with them. "You've got steel in your soul, enough to carry you through." When he'd said it, Porthos had thought that it would be possible to find a home under this man's command.