I have been watching the OVAs and I planned to read the manga as soon as I find where the story leaves off at the anime's ending. Anyway, Yomi/Yami has been targeting Kenchi because he's the disciple of Ryōzanpaku which makes him the strongest disciple in History. Can everyone tell me why is that, why Yami has a feud with Ryōzanpaku, and killing/defeating Kenchi would do? Please explain in detail.
History's Strongest Disciple Kenichi Wiki