Like King Pistachion and the other pistachions, Derek is bent on world domination. He spent seven years alone brainstorming his overthrow of the humans before carrying out a plan that would take fifty years to complete. The plan he created relied strongly on patience and stealth.
In spite of his dedication to carrying out his father's plans, Derek took a summer off to do some modeling. He calls himself young and foolish for doing so but claims he has no regrets. Derek became so focused on monologuing his evil scheme that he failed to notice his prisoners escape, much to his frustration. Afterward, he simply resorted to full out attacking them, attempting to destroy their time vehicle.
Like the other pistachions, Derek is a large humanoid pistachio plant.
Disguised as an intern, he looked like a light skinned human with short brown hair and black, thick-framed glasses. He wore a white collared short-sleeved shirt with a blue-grey tie and a pair of pants with visible seams and a black belt. His shoes were brown loafers with darker laces and white socks.
Afterward, he donned a disguise to look identical to Orton Mahlson wearing his Doctor Zone outfit but soon removed the mask, keeping the costume. After removing the gift box, his right hand was exposed as plantlike, while his left hand remained human-like.
In Fungus Among Us, Cavendish, Dakota, and Milo met Derek in 1965 after Milo got a call from Dakota (in the present). They wondered how he remained after Diogee peed on his father's seedling. So Derek explained his backstory and his own plan for overthrowing the human race. Back in the present, his plan seems to be working.