Characterisation is quite obvious. Yoshiya is the protagonist, and his inner demons, turmoil and lack of knowledge about his origins are the antagonists.
It is interesting that Murakami chooses such an average man to explore these concepts, and the idea that God may have chosen another woman to conceive and give birth to a child without the aid of a man. This is, however, very much in keeping with Murakami’s habit of exploring wild ideas through the most unexceptional of people.