His Eyepatch holds back his intense reatsu, and when he takes it off, he explodes with power. It was shown that just the reatsu coming from him cut down a building. Kenpachi has always been insanely strong, beating Ikkaku when he was still a trainee in Rukangai. His Vice-Captain, Yachiru, seems incredibly attached to Kenpachi since he trained her from childhood. She always tells people not to bother her or Kenpachi when Kenpachi is fighting.
Kenpachi is so intensely in to fighting, he has been described as a "Demon" by Tousen. Kenpachi took out both Tousen and Komomaru, even while Tousen's Bankai kills all your senses, Kenpachi still managed to beat Tousen.