Before I get into this, I want to point out that I think Bleach is a really lousy anime. The story goes haywire all the time, nothing gets me invested into the plot, and the only entertainment value comes from finding a character you want to invest in, learning about the lore, and enjoying the fight scenes.
With that said, I really like Hisagi Shuuhei. Though he's not a major character, he did get that one episode that fleshed out his character nicely, and I gotta say, I was really impressed.
The first thing that caught my attention was the contrast between his outwards personality, and his zanpakuto. Kazeshini is a brutal and cruel weapon: it's forte is in binding opponents, catching them off-guard with sneak attacks, and all other kinds of under-handed tactics. Shuuhei himself remarks on how he hates his weapon because it's not only functions as a device of cruelty, but the very look of it flat out freaks him out.
When you consider that the Zanpakutou is the physical embodiment of a person's soul, you have to wonder how this unfair and murderish looking tool could possibly be an extension of such a cool-headed, hard-working and seemingly honest guy.
As his character is further exposed, I realized that deep down inside, Shuuhei might be a sociopath. He's had to deal with so many failures in his life, from sitting helplessly while his friends were about to be butchered to watching his squad mates die under his command, but his inexpressive personality bottled all that frustration inside of him and created something freakish that even he doesn't know how to deal with.
Taking all that into consideration, where does his strong sense of morality come from? He's well-liked by his peers, puts others before himself, and genuinely puts a considerable amount of hard work into everything he does.
Then I found my answer: it's the people around him, the role models, that inspired the good in him. Kensei Muguruma, who saved his life when he was a kid and inspired him to become a Shinigami. Kaname Tousen, who was his mentor and teacher for a majority of his Recruit years. Izuru Kira, his best friend. All these people were good to him, helped him grow, taught him to stand on his feet and helped him build a strength of character that can overcome that darkness inside him. Now he uses it as a weapon to save others.
I'll give Kubo this, while he sucks at story-writing, he does make some pretty awesome characters.