Growing up Mugen knew words like hunger, fear, and alone. As a teenager lust and survive were words he became intimately familiar with. A thief in the Ryukyu Islands didn't need a large vocabulary.
He threw around profanities like fists to accompany the thrusts of his sword. He let them know that he was no one to be messed with. He let his fighting speak for him, always impressed with how articulate a knife through the gut could be. Violence didn't need as many words as peacefulness seemed to. He never understood the need for idle chatter when there was nobody to kill or rob. Those times were better used for drinking and fucking.
Women were another story altogether. Words seemed to be the only thing that could help him, aside from his charm and roguish good looks, of course. He could spew beautiful lies about a woman and use whispers and quiet encouragement like soft weapons. The vocabulary required to fuck was what he referred to as The Other Side of the Sword.
Fuu and Jin forced him to talk. Especially her. All she did was rattle on and on about sunflowers and samurai and food. His interest would peak at the mention of anything edible, but he'd zone back out when he'd realize she was just reminiscing again. His replies to her many questions about his past were obviously unsatisfactory. He was a thief, not and idiot. He could tell when she asked the same thing twice but reworded her query. He foolishly did not think before he spoke with her, often resulting in arguments and her calling him a jerk and stomping off. But there were a few times when something he would say would make her eyes all shiny and turn her cheeks pink. Times when something he would say would actually make her silent. Try as he might, he could never pinpoint what it was that caused this reaction.
After a while, he grew accustomed to her rambling, even missing it when she was off doing whatever (usually getting herself kidnapped). He'd learned many new words from her about feelings and emotions. None of which he'd ever utter aloud, but it helped him understand her better if he knew what the hell she was going on about. When he bothered to listen, that is.
Jin was the loudest quiet person he'd ever met. Polishing his sword by the campfire, Jin would look at him and silently talk a blue streak. Mugen's eyebrows would furrow and he would feel his eyes crossing as he would concentrate on deciphering the ronin's unspoken words. During their travels Mugen became an expert at understanding which sighs meant that Jin was annoyed but wasn't going to do anything other than sigh loudly, and which sighs meant that Mugen had better duck if he wanted to keep his head. The 'hm's' and 'mm's' and 'hn's' that were the most common form of communication that Jin offered (to Mugen, at least) often left him shaking his head and doing his own thing, regardless of what Jin had just hummed about.
It wasn't long- quicker than he'd like to admit- that Mugen spoke fluent Jin and Fuu. In the beginning he'd picked up patience and share. Goodbye was a word he was too familiar with but he refused to let it be part of his FuuJin Dictionary. So in the end, as they parted ways, he whispered his newest words like a mantra; trust and friends.
Days later, sitting at a bar muttering drunken slurs under his breath Mugen realized they had a word. One word, one syllable, one word that he could actually read and write. One small word that was a summation of his time with them. Happy.