Professor Elvin Gadd, often shortened to Professor E. Gadd or simply E. Gadd, is the scientist first appearing in Luigi's Mansion and founder of Gadd Science, Incorporated. He always makes ambiguous gibberish sounds when speaking, and he frequently uses phrases such as "criminy," "hullabaloo," etc. His name comes from the archaic exclamation "egad," something that one says in surprise.
In his past, E. Gadd lived on the foothills of the Thwomp Volcano, as depicted in Mario & Luigi RPG 2: Partners in Time. The Mario Bros. go back in time and end up saving his lab from burning down when fire blocks erupted from the Thwomp Volcano. He then helps the Mario Bros. by inspiring his older present self to create the Hydrogush 4000; as the Mario Bros. are in the present, he starts to get the idea for the device, which transfers to his future self. The present-day Professor E. Gadd uses the Hydrogush 4000 to send water through via Time Hole and calm the erupting Thwomp Volcano. Afterwards, the twenty-year-old E. Gadd moves to Boo Woods, where he would later study ghosts. In order to get funding for his ghost research, E. Gadd also started a cafe franchise called Starbeans Cafe.
Many decades later, in Luigi's Mansion, Luigi enters the mansion and is saved from ghosts by Professor E. Gadd wielding the Poltergust 3000. The professor reveals that he lives in a house near the mansion, where he researches his favorite subject: ghosts. Luigi explains his brother Mario is missing within the house, and the professor decides to help Luigi by giving him the Poltergust 3000 and the Game Boy Horror. The professor continues to help Luigi throughout the game, giving him help and tips.
After a few years of absence, Professor E. Gadd appears in Luigi's Mansion: Dark Moon. In the game, E. Gadd takes up residence in Evershade Valley after learning that the ghosts there are friendly. Eager to examine ghosts without fear of being attacked by them, he sets up a laboratory in the Gloomy Manorand a Bunker close by the mansion. There, he begins researching the ghosts of the five mansions that populate the valley, and sends his five Toad assistants to the other four mansions in the valley to examine the ghosts there. At first, the resident ghosts welcome him and assist him in his lab, but when King Boo arrives and destroys the ghost-pacifying Dark Moon that hangs over Evershade Valley, the ghosts start running amok and destroy E. Gadd's equipment. He flees to his nearby Bunker and enlists the help of Luigi - who has now taken up residence in his new Grade D mansion from the previous game - to help rebuild the Dark Moon and calm the ghosts once more. E. Gadd was later revealed to have been unintentionally and indirectly responsible for King Boo's return, as he had sold his portrait in a garage sale, thus ensuring his escape.
In this game, E. Gadd has made an upgrade of his Poltergust 3000 from the last game: the Poltergust 5000. This new Poltergust uses a Strobulb to stun ghosts, rather than the flashlight used with the Poltergust 3000. Professor E. Gadd also introduces three new inventions: the Pixelator, a machine that turns Luigi into pixels and transports him to and from the different mansions, the Dual Scream, a device that resembles a Nintendo DS and is capable of communication, and the Dark-Light Device, which allows Luigi to see invisible objects. He doesn't leave the Bunker until the end of the game, directly after beating King Boo.
E. Gadd has a similar role in Luigi's Mansion Arcade to that of Luigi's Mansion: Dark Moon, in that he communicates with Luigi while the latter explores the mansions, and offers a tutorial to the player upon starting the game.
E. Gadd returns in Luigi's Mansion 3. Having been tricked by Hellen Gravely into coming to the The Last Resort, E. Gadd is trapped in a painting and his ghost collection, including King Boo, is released. He is later saved by Luigi, who escaped a similar trap and borrowed the Poltergust G-00 from E. Gadd's car in the hotel's basement. E. Gadd initially attempts to flee the hotel, but when Luigi insists on staying to save his captured friends, E. Gadd relents. After setting up his portable lab in the basement, E. Gadd gives Luigi the Virtual Boo for communication purposes. He later has Gooigi accompany Luigi throughout the hotel. He also uses the lab to provide a safe haven for the saved Toads.
Towards the end of the game, King Boo attacks Professor E. Gadd's lab, and traps him and everyone else in a single painting. Luigi was saved from this by Polterpup, and he manages to recapture King Boo and free all his friends. He also offers to help the ghosts rebuild the then-recently destroyed Last Resort complex, providing construction hard-hats to Luigi and some of the ghosts. After the hotel is rebuilt, Professor E. Gadd stays behind with Gooigi and the ghosts while everyone else departs.
Although Professor E. Gadd does not appear in Super Mario Sunshine, when Mario initially finds F.L.U.D.D. at the Delfino Airstrip, it says, "Thank you for purchasing this item from Gadd Science, Incorporated" Later, Bowser Jr. says he got his Magic Paintbrush from "a strange old man in a white coat." Bowser Jr. uses the paintbrush to frame Mario and get him away long enough for himself to go in and kidnap Princess Peach. This is a fitting description of Professor E. Gadd, which is further confirmed when F.L.U.D.D. seems to react in surprise to the statement. Bowser Jr. claims E. Gadd gave the brush to him of his own free will and that he did not know the professor's name when he received the brush, but it is unknown whether this is true or not. There is a symbol on both the Magic Paintbrush and F.L.U.D.D. showing a face and hairstyle similar to E. Gadd's, which seems to be the Gadd Science, Inc., logo. It appears on many of E. Gadd's other inventions, as well as the boxes for F.L.U.D.D.'s nozzle upgrades. This is the only mainline Mario game where he is referenced in any way.
His normal self is voiced by Christopher Lloyd and his young self is voiced by Ben Diskin in the English dub and both selves are reprised by Kazumi Totaka in the Japanese dub.