Fuu
little sunflower girl
Fuu is the reason Jin is traveling -- he and Mugen are bound to walk with her until she finds her mysterious Sunflower Samurai. What makes this quest frustrating for both of the boys is how vague Fuu is about the whole thing. Her only lead is that this man "smells of sunflowers"; as it turns out, this sole clue is derived from her last memory of him. Fuu never explains this to either of them for unknown reasons.
We know from the start that if it wasn't for Fuu, Mugen and Jin would probably be locked in an endless sword duel for the next five years. In the beginning, she's the sole reason keeping them from killing each other, the sole reason why they walk (semi) peacefully in this oddball little group instead of wandering on their own. Both Jin and Mugen see their promise to accompany her as just that -- a promise. There is no other motive behind it; they'll fulfill their obligation, then go their separate ways. Nevertheless, despite their initial indifference to Fuu's goal, both boys come to care for her like a little sister.
Jin is always calm and respectful in front of Fuu. Even when he's actually annoyed with her (which, if I may note, is rather rare) he barely shows it. At first, however, he isn't interested in talking with her, stumping conversations with his usual grunts. At most he seems indifferent to her outbursts and extroversion, knowing that he has endure it until he's come through with the promise. Nevertheless, he does start to show concern for her on several accounts to point of surprising her -- when the gang split up in Episode 16, he seemed to be the only one bothered, maybe even disheartened. When he hears Fuu scream in the distance, an expression of worry flashes across his face for a brief second; I wouldn't doubt that if he weren't preoccupied with Yukimaru, he would have gone to her rescue.
When Fuu screams again, Jin rushes up to her, asking what happened -- he does this even though he knows that she is pissed off at him and Mugen. At first Fuu responds disdainfully, but stops when she realizes the sincerity in Jin's few words. When the two hear that Okuru -- the man who saved Fuu's life -- is being pursued, Fuu is the first to go to his aid. Jin compliments this, saying that she is a "woman with a sense of duty". You'll notice that he no longer responds to her queries with impassive "hm"s -- in fact, he almost seems mildly dismayed when Fuu speaks coldly towards him.
But the best example of Big Brother Jin has to be the little moment the two share in Episode 24. He gives her a chance to speak to him, a chance to let out her feelings -- whereas, if you recall, it was pretty much the other way around in the beginning -- and even allows her to give him a hug. The surprising thing about this is how Jin sort-of reciprocated by putting a hand on her shoulder. It may not seem much, but for Jin it is -- it shows just how much he has come to care for her as a friend and as the person he's promised to protect.
Although there is basis for it, I don't necessarily see Jin and Fuu's developing relationship as romantic; rather, I think it is a sibling-type bond. Jin is like many older brothers in that while aloof, underneath he truly cares about the well-being of his 'little sister'. He becomes someone she can look up to and lean on when she's not feeling right -- something I find infinitely more sweet than any romantic relationship between the two.