Slow
It has become somewhat of a mutual phobia, between them. They have both died, and in water. Both involved the retribution of the past upon their present lives.
They both came back because there was finally a reason to.
Nothing showed of it except relieved glances and a few quiet days that culminated in epic fights and a shouting, fustrated and misunderstanding Fuu. A reassurance that, yes, I will be the one to kill you, nothing else. A reassurance of strength that they can they can rely on. The violence is a medium they can understand each other through.
When Mugen is sucking on a bruised finger, tounge exploring a broken nail, there is an easy slouch to Jin's shoulders and sometimes their fingers meet when reaching for the same dango. When Jin's hair is down (hardly ever; never), Mugen's eyes linger and can't meet glassed eyes. They don't know how to go about this, and so it remains adolescent, all fumbling and sideways glances.
Fuu does not understand their insistance on glacier-slow movement. She thinks it is romantic, cute, and therefore there is nothing to hold them back. They almost envy her enthusiasm for love (or is it?), contrary to their own jadedness, especially after it completed her usual mourning cycle of wails and sulking. But underneath that, she cannot help but feel protective of her older brothers and their fragile hearts.
Jin understands her desire to bring them together, but does not see a need for haste. But his knowlege of his own thoughts brings with it the knowlege that he is most likely just using this as an excuse to hide the fact that what the real problem is, is that he's scared. Shy. Intimidated. None of the words that fit make it sound any better.
Mugen doesn't like the feeling that comes with the desire. Desire, sure, he could deal with that. It was the feeling that they were both children who had grown up too fast, but that he was the one who had stayed a child. It was the fear of the hurt of rejection, and Mugen didn't like to think that he was the kind of guy who couldn't jump off a cliff 'cause he was scared to hit the stones, if meant his life. It's just that Jin doesn't fit into any standards that Mugen has learnt to apply to people, and just because everyone else in every other category have proved worthless, maybe this is the one category that isn't.
And so, at the speed of a gluttonous snail against a large headwind, they move forward towards each other.
