Grandpa Quinzel is the grandfather of Harley Quinn. He and his wife, Harley's grandmother, seemed to have a good marriage and gradually grew disdain and hatred for one another over time, as seen in the sitcom-parody sequence showing moments in life of Harley and her family when she arrives at her childhood home. He is shown to have hated his wife so much that he offers full-on celebration complete with alcohol to drink upon hearing that she had died. He was known to have high blood pressure and died after being shot with a through his entire torso by the assassin hired to kill Harley as presumed collateral damage. It is unknown if he knew of the plan to kill Harley for the bounty that Harley's parents had acted upon, but it is likely he did not.
Grandpa Quinzel seems to have a very strong hatred for his wife despite their once stable relationship. He also appears to truly love his granddaughter, Harley, shown with how happy he was to see her when he showed up to their family home. He presumably opts to celebrate by drinking.
Grandpa Quinzel is bald with only a light amount of grey hair left on the sides of his head, he has very bushy eyebrows and wears black, thick-rimmed eyeglasses with light blue lenses. He seems quite skinny and is seen wearing alight green collared shirt, brown pants that are pulled up high and fastened with a black belt.