Jekyll created a machine that could turn someone into either a fairy princess or a monster. When the angry Mob 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 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 heavily implied that the Platypus monster has eaten him when the monster coughs up his fairy wand ("The Monster of Phineas-n-Ferbenstein").
He is obviously a parody of Dr. Jekyll from the famous story Strange Case of Dr Jekyll and Mr Hyde. With his Name Jekyll, his alter-ego, and "Heinz Doofenshmirtz" having a similar name.
Unlike his descendant, Jekyll is right-handed.
Although it was implied that the platypus monster ate Jekyll, he most likely survived as he appeared unmarried and was an ancestor of Doofenshmirtz. However, he could have been married at the time he was eaten, and his wife just wasn't present, or he was divorced like his descendant.