Jekyll Doofenshmirtz is Heinz Doofenshmirtz's great great grandfather who created a machine that could turn someone into either a fairy princess or a monster. When the angry Mob (A crowd of angry people) showed up at his house for their appointment, he drank the concoction the machine produced to show off the invention. However, the dial was turned to fairy princess, thus ruining his chance to be a real evil genius. The mob left, annoyed with the supposed failure of Dr.Jekyll's invention. Realizing the problem, he quickly changed the dial to monster and drank the concoction. Now a monster, he ran through the village spreading evil; however, during his reign he ends up upsetting the platypus monster (A creation of Dr. Phineastein and Ferbgor). Later, at the monster ball Dr.Jekyll is back to normal but ends up meeting the platypus monster again; immediately, the monster begins to beat him up. After asking for a time-out to go get a drink, Jekyll once again drinks his invention's brew, but ends up turning himself into a fairy princess. The scene changes, but it is implied that the Platypus Monster has eaten him. ("The Monster of Phineas-n-Ferbenstein")
- He was seen in a story told by Dr. Doofenshmirtz.
- He is a parody of Dr. Jekyll from the famous story Strange Case of Dr Jekyll and Mr Hyde.
- He is Doofenshmirtz´s great great grandfather.
- Unlike his descendant, Jekyll is right handed
- Although it was implied that the platypus monster ate Jekyll, he either must not have, or Jekyll survived, because he appeared unmarried in the episode, and was an ancestor of Heinz.
- Although Dr. Jekyll is only Heinz's great great grandfather, Ferbgor is Ferb's great great great great great (etc.) grandfather.