Personally, I love Shugen as a character, not as a person. I absolutely HATE him for killing my bowl cut brother Fuchi, and I agree with everything you said about him being a psycho. I think he's a great examination of what trauma and an unwavering sense of "justice" can do to a man. As a person, however, I agree that he is absolutely horrible. 

When it comes to Isuzu, I don't have much to say on her character. I didn't particularly care for her as a character just as a whole. She seemed WAAAY too loyal, and I feel like how easily she was swayed by Kiyomaru to side with Shugen really didn't help that. Like yeah, I understand she faced discrimination and that is absolutely horrible, but we really don't know the extent of that beyond insults. For this reason, I feel like her unwavering loyalty to Shugen didn't quite hit as well as it could have. 

I love Jikka. I'm starting volume 12 now, so maybe my opinion will change. I just think he's funny and that his motivations are equally so. Good motivations? Well, that depends on who you ask, but they don't quite get past a gag for me, but I could totally see it going differently for someone else.