Complete Monster: Johnnie Walker - mutilator of cats (and he says that the cats are paralysed but can feel everything he does to them, and that the pain is "necessary"), manipulates Nakata into committing murder and it's strongly implied he's Kafka's father, who told him over and over again he would kill him and then have sex with his mother and sister. From when he was a kid.Jesus.
Fridge Logic: Borders on WMG due to the nature of the book, but on a second reading once can infer the following; Kafka's dad? Johnnie Walker. Whether he was born/created as Johnnie and hid it well, or Johnnie took over his body recently is anyone’s guess.
The Woobie: Nakata and Miss Saeki are the two most obvious examples, but Kafka has moments too.