Personality
Kurogane has a straight-forward, strong, moody temper. He's definitely not fluffy or cheerful, and neither he will be in a near future. Or ever, maybe.
He's dead serious, sometimes only showing borderline psychotic emotions like rage, so that makes it funny when he's constantly abused by Mokona and Fay's antics. However, he's not just your average pissed-off ninja character with blood-thirst. He's deeper than it seems. When you get to know it, he's responsible, loyal, strong-willed, noble, proud –hence his annoyance when he's teased-, and he does care about other people when he gets to know them better. Sure, he's a murderer and a fighter, but he has a lot of qualities and moral principles defining his personality. I've always thought of him as a samurai, following the path of a warrior in a bushido kind of way but being faithful about his mistress all the way, even if she “turned her back on him.”
I hate to do the comparison, since it's cliché, but he really is like a dog. A fierce watchdog, that is. He will bark and bite you, sometimes because you pissed him off, and sometimes just for the heck of it, but he'll be nice with his owner and with the ones who treated him right. Or the ones he got used to. You won't see him waging his tail or licking you, though, but he has his own methods to show you he's pleased.
His emotions are always transparent. Kurogane's facial expressions and predictability are fascinating. He's not going to hide you stuff, but he's not going to happily chit-chat about his life, either. And you can't blame him. I mean, in his place, who would?
And there's his evolution as a character. First, he's just a creepy, violent man who's forced to travel with two kids, a weird guy and a creature over the dimensions. He doesn't care about anything, as long as they get over with everything quickly so he can come back home. He lets them know right at the start he's not going to play around or be concerned; and he's just there because he has no choice. He does it for himself.
But as the time passes by, he seems less rude. While he's not entirely friendly, he lives the moment and spends time with his companions, and enjoys the few things he can. Sometimes, he's even behaving dorkily. For instance, he buys shounen manga in order to read a ninja series he got addicted to in Hanshin. He competes with Mokona over the food. He joins Syaoran in the oni-hunting at Outo because that was fun for him. He drinks booze because he likes it… although he noticed it wasn't such a good idea to share it with the others…
And then, since Outo country, we see him more adapted to the group. The drinking issue, for instance. He's always trying to make sure the other ones won't drink a lot, since they start behaving like idiots. But more than embarrassment, it's more like a big brother feeling of taking care of them. To the point he's even nicknamed “daddy” XD He teaches Syaoran how to use the sword. He tried to understand Fay and his weak excuses, even though they're not his cup of tea. He feels offended when Fay lies to him or hides things from them without explaining himself. He's still mad at the nicknames and abuses, but that's to be expected.
He's growing up as a person and a character. We get to know his true essence. I couldn't know if he's truly getting attached to the rest of the group, but I'm certain he doesn't mind them so much anymore.