Dr. Hogback was a renowned physician who, according to legend, saved as many lives as there are stars in the sky. He vanished from public view, causing uproar among medical professionals. Nami, Usopp, and Chopper encountered him on Thriller Bark where he disclosed that his current research focused on understanding life and death through the use of zombies present on the island.
This is later exposed as a lie when the crew discovers that Hogback himself created the zombies, and is a crew mate of Gecko Moria, who breathes life into Hogback's zombies with the shadows he steals, transferring the fighting ability and personality of the shadow's former owner into the body. He is later seen meeting with Gecko Moria, Perona and Absalom concerning the creation of Zombie #900 and the removal of Luffy's shadow.
After facing the betrayal of his beloved zombie version of Cindry, Chopper and Robin prepare to finish him off with a suplex wrestling attack, but the rampage by the massive Oz prevents them from finishing the attack. As Hogback tries to speak with the giant zombie, Oz squishes him under his massive foot. It's shown, however, in chapter 483 that Hogback somehow survived from Oz's attack.
After Chopper and Robin being defeated, Hogback and Chopper have a conversation, where Hogback reveals that he never cared about saving lives and the reason he went into hiding was because so many people were coming to him seeking treatment, and that he became a doctor only for the money. Robin outsmarts him by telling him to order Jigoro and Penguin to jump out the window.
Hogback faced off against Chopper, who, as a doctor, resents his experiments in zombification for being intended for personal gain rather than the progress of medical science. However, much to Chopper and Robin's dismay, Hogback had prepared Jigoro and Sanji's zombie for the battle to come.