He is portrayed as an elderly, mustached man who wears a black suit and top hat.
Much like his granddaughter, he lacks empathy for other living beings. He isn't above torturing his own adopted son, and when he does display some affection for him between torture sessions, it's clear that it's to manipulate him into staying within his grasp. Considering the fact that he often strips and whips Shiro, it's safe to assume that he's a pedophile.
In the manga
Genjurou only appears in flashbacks, as hed died several years prior to the main events of the manga. Originally an acquaintance of Bizen Chiharu, an aspiring doctor, he founded the Imari Ishin Facilty when his granddaughter discovered the pair of larva monsters. Around that time, he had also adopted an orphaned Shiro, and raised him as his own.
Seeking inspiration for his paintings, Imari abused his adopted son for an untold number of years. From locking him in his house, to stripping him naked and whipping him severely, Genjurou made the defenseless child into his plaything who felt sexual gratification from destroying his innocence. Throughout the torture sessions, Shiro believed that him being tortured by his adoptive father meant that he loved him, thus fueling his "pain is love" ideology. It's also heavily implied that Imari performed horrific experiments on Shiro, which explains his sheer strength and agility.
Eventually, Genjurou committed suicide by hanging himself, and Shiro escaped. Unfortunately for Shiro, the damage was already done, and he became the abusive father that Utsutsu came to resent.
- Due to the anime only covering the first portion of the manga, Genjurou is excluded from the anime adaptation.