He's on trial for his life for most of the book, accused of murdering his old buddy Carl Heine, Jr. over a land dispute between their families. Like Carl, Kabuo is the strong, silent type, and the war and its aftermath have definitely caused him to build up some walls (psychologically speaking) that even his wife can't really see past.
Right before the birth of their first child, for example, when Kabuo is obsessed with the loss of his family's strawberry land, Hatsue is at her wits' end trying to figure out how get his mind out of the past. She hopes that having a family will help snap out of it, telling him, "You're going to be a father soon [...] I hope that will make you happy, Kabuo. I hope it will help you bury all of this. I don't know how else I can help you" (25.11). Of course, the issue of the strawberry land comes up again a few years later, right before Carl Heine is found dead and Kabuo ends up on trial. So, no, that past doesn't really get put to bed in the way that Hatsue had hoped...
While Kabuo's father, Zenhichi, made an effort to be deferential and ingratiating to the Heine family, Kabuo is less like that. When he returns from the war, he lets his disappointment and anger show when he realizes that Etta Heine sold off some land that was supposed to belong to his family. The coldness that he can sometimes give off when disappointed or frustrated does little to endear him to others.
In fact, because of his calm poker face, the people watching the trial find Kabuo haughty and aloof—and, as a result, unsympathetic. Of course, their big problem with him is that he's of Japanese extraction (since they are, you know, fairly racist as a group).
Unlike his wife, Kabuo does not seem super-indignant about the fact that he's been unjustly accused of murdering his old friend. Instead, shockingly, he seems to kind of feel like he deserves it. Because he killed people as a soldier fighting for the Allied Forces, he already thinks of himself as a murderer. This secret feeling seems to color his attitude toward the predicament he's currently facing.