The character we are looking for is a geneticist who tries to find a formula to regrow missing limbs. As fictional scientists often do, he tests his invention on himself—with decidedly unsafe results.
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Dr. Curtis “Curt” Connors is a surgeon and geneticist in the Marvel movie and comic-book universes, where he appears as an antagonist of Spiderman. Connors lost his right arm in a war and is trying to find a method to regrow the missing limb.
Inspired by the regeneration capabilities of reptiles, he creates a serum based on lizard DNA. Tests—first on animals and then on himself—lead to the successful replacement of his arm. However, as often happens to fictional scientists, the serum has an unwanted side-effect: Connors morphs into a lizard-like monster, known simply as “Lizard”. He also develops the megalomaniacal plan to convert the city’s entire population into lizards.
In the movie universe, Connors and his plan are eventually stopped by Spiderman (and his love interest Gwen Stacy), and he is ultimately cured. In the comics, the cure is not complete and the Lizard part of Connor resurfaces time and time again. He fears that the Lizard will one day completely take over and works tirelessly to find a permanent cure.