I like a lot of the minor characters, they're part of the richness of Hikaru no Go.
Ashiwara, Kurata, Ichikawa, Hikaru's grandfather, Mitani's sister


But its not neceassily the most likeable that I like the most.

Nakamura (toad man), Akota (wig man) and Dake (the guy who's called in to deal with Mitani's cheating) are all rather grotesquely facinating. They're not real 'villans' - though Nakamura's getting close ^_^