Gustav Sebald
Dr. Gustav Sebald was Dr. Montgomery Montgomery's herpetological assistant, who aided him in his research and in the construction of reptile traps (jobs that were later given to the Baudelaires, orphans sent to stay under the doctor's care) so that Dr. Montgomery (called "Uncle Monty") could acquire new specimens for his reptile collection.
Aside from acting as Uncle Monty's top assistant, Dr. Sebald was a little-known film director, known only to the V.F.D. for such obscure films as Werewolves in the Rain, Vampires in the Retirement Community, Realtors in the Cave, and most notably, Zombies in the Snow, all of which are suggested to have secretly included warnings to V.F.D. members, encoded in the dialogue. The code, known as the Sebald Code, is his own creation. The code is detailed in Lemony Snicket: The Unauthorized Autobiography.
Soon before the Baudelaires’ arrival at Uncle Monty's residence, Count Olaf abducted Dr. Sebald and either forged or had forged a note of resignation to be given to Uncle Monty. He then proceeded to drown Dr. Sebald in the nearby Swarthy Swamp. Due to this, Dr. Sebald never actually met the Baudelaire orphans and so never directly appears in the main series.
Count Olaf, under the guise of Uncle Monty's new assistant Stephano, took Dr. Sebald's place in the Montgomery residence (a position which apparently required a great deal of responsibility and, on Monty's part, a great deal of trust) eight days after the Baudelaires first arrived. In accordance with the doctor's frequent habit of taking the children to a movie, the children, Olaf, and Uncle Monty went together to see Zombies in the Snow at the nearest multiplex (in the town of Tedia).
In the film it shows Sebald chained to the front of a moving train (probably because of Olaf).
Read more about this topic: Fiona Widdershins, Members of V.F.D., G.