Professor E. Gadd, (sometimes referred to as E. Gadd; known as "Professor Oyama" in Japan) is a character who has made a number of appearances in different Mario games, each time appearing as a mad, but friendly, gadget-inventing scientist. His full name is Elvin Gadd. He is apparently left-handed or south-pawed, as shown in Mario & Luigi: Partners in Time when he is shown operating a Nintendo DS with a stylus in his left hand. He was also mentioned in Super Mario Sunshine, being the creator of F.L.U.D.D..
In Japan the professor is named after Nintendo game designer Yoshiyuki Oyama. The English name is a play on the minced oath egad! or egads! His speech pattern consists of specialized gibberish, but the translation (or his "real words") are always in a text box.
E. Gadd debuted in Luigi's Mansion as one of the main characters. Luigi enters the mansion and is saved from ghosts by a strange old man wielding a vacuum cleaner. Professor E. Gadd 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 two of his inventions, the Poltergust 3000 and the Game Boy Horror, the professor continues to help Luigi throughout the game, giving him help and tips.
Super Mario Sunshine
In Super Mario Sunshine, when Mario finds F.L.U.D.D., it says: "Thank you for purchasing this item from Gadd Science, Inc." Later, Bowser Jr. says he got his magic brush from "a strange old man in a white coat". 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. There is a symbol both on the Magic Paintbrush and F.L.U.D.D. and its upgrade boxes showing a face and hairstyle similar to E. Gadd's, probably the Gadd Science, Inc. insignia.
Mario & Luigi series
In Mario & Luigi: Superstar Saga the professor makes an appearance supplying the brothers with an array of accessories to help them along their way, after they try each drink at Starbeans Cafe, a coffeehouse that he owns.
In Mario & Luigi: Partners in Time the professor debuts his newest invention to the Mushroom Kingdom, a time machine. However, when Princess Peach ends up stuck in the past, E. Gadd provides the brothers a suitcase enhanced with artificial intelligence named Stuffwell to help them on their quest. The heroes later meet a younger Prof. E. Gadd (thought by the Toads there, in the English version, to be named interjection-like names, such a U. Reeka & Yao Zerr), who is researching the behavior of the Thwomps at Thwomp Volcano. After his lab is destroyed by the eruption of the volcano he mentions his interests in studying ghosts, and that he would be buying a lab at the edge of Boo Woods, which is the likely location of Luigi's Mansion.
In Mario & Luigi: Superstar Saga + Bowser's Minions the professor have an additional appearance in the Bowser's Minions mode where he is seen doing research in a cave full of Boos, he explain his research to Captain Goomba but he doesn't bother to lisen to him and leave, E. Gadd continue his speech as if he didin't noticed the Goomba leaving and explain that he is also working on a time Machine, likely refferencing the events of Partners in Time.
Mario Party series
In Mario Party Advance, the player can use the coins earned in the mini-games to buy Professor E. Gadd's zany "Gaddgets". In Mario Party 6 Professor E. Gadd appears in his own board, E. Gadd's Garage. Players see him when they land on a ? space and usually get to try one of his inventions, like a giant fan that sucks the other players coins away.