I would say I like just about every character, including Kyuzo. I just don't find him as crucial to the film as these other arcs mentioned. The scenes I consider non-crucial amount to a very insignificant amount of run-time when all totaled-up.
I would love to read anything written by Kurosawa. I will look for that book at an affordable price.
Do you have the autobiography? If you do not I would consider that a must purchase. The only annoyance is that he stops it basically after Rashomon. The interview books covers whatever the interviewers ask him. There is much to glean from this, but like the Keaton interviewer book much material is covered several times (this particularly annoys me because I would have loved to asked more in depth questions -- both Kurosawa and Keaton). But you can see what sources I normally use at the bottom of my Yojimbo review. I bought the Stephen Prince book, but have not read it yet. I am debating on getting the Stuart Galbraith book.