Well that was fucking difficult. And it's not just the cheesy plot.
I've asked Unni about it, and agree with her when she says that fandom puts more evil in Yagyuu than Yagyuu might actually have within him - or, we like him evil. I reread the manga bits, then read the fanbooks, and somehow the two don't really match up. Then I got the horrible feeling that I didn't know who the fuck Yagyuu really is.
So in the end, I made Yagyuu someone who does things because he needs it done (to forget), despite knowing the consequences (being cruel to the niou). He put himself before others (probably typical of an upper class), he was competitive and was willing to take on challenges (living with Niou in the slums, going to travel with him - although that didn't happen in the end)... and I don't remember what I was going to say. Someone, come poke my brain and make it work.