Marvin is a character in the 37th Give Yourself Goosebumps book, Escape from Horror House.
He is a ghost hunter who uses technology for his job instead of supernatural powers.
His main rival is psychic medium Madame Matilda.
If the reader chooses him over Matilda, his technology will detect that there are actually no poltergeists in your home, and psychic, telekinetic energies have been found instead. Marvin will then reveal that either you or your sister Jen have telekinetic abilities and are causing the weird psychic energies that are messing up your home.