Derek is an antagonist in Milo Murphy's Law who is the son of King Pistachion and serves as the main antagonist of the episodes "Fungus Among Us" and "The Phineas and Ferb Effect".
Background
In Missing Milo, he found a way on top of the Time Limo inside the Time Stream as Milo, Cavendish, and Dakota escaped from King Pistachion. When Dakota discovered Derek on top of their vehicle, he knocked him of the limo with a big sack of clocks.
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.
In The Phineas and Ferb Effect, he monologues in front of Melissa, Baljeet, Zack, and Buford about how he tried to run the Doctor Zone franchise himself back in 1965, but failed. When the kids run away from him he orders his army of Pistachions to chase after them. His army fails at first, but eventually they catch up with them and take Milo, Phineas, Ferb, Melissa, Zack, Baljeet, and Buford. However right when Derek is about to turn the kids into plants, Cavendish, Doof, Dakota, Orton, and Perry return from there adventure and cause a fight between Dakota's other versions of himself and the Pistachion army.
Throughout the confusion of the battle Milo, Phineas, Ferb, Cavendish, Dakota, Orton, Perry, and Doof make it up to a high point and call upon Professor Time, Doof's future self. Professor Time calls Orton to come with him to 1955 to destroy Derek before he started to hatch his master plan. Orton and Professor Time complete their goal and Derek and his monster Pistachion army fade away and die.
Personality
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.
Physical Appearance
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.
