Professor E. Gadd is a mad scientist with a taste for adventure. First appearing in Luigi's Mansion on the Nintendo GameCube, and then become a helpful ally in the Mario and Luigi games.
In Luigi's Mansion he was a ghost hunter. He lived outside the mansion in a large hut where he could turn the captured ghosts into paintings, but unfortunately the ghosts broke out due to the evil King Boo. He went around the mansion taking down the ghosts with his trusty Poltergust 3000. But he had lost his youth and edge, enter Luigi. He found Luigi wandering in the mansion searching for his brother Mario. It turned out that after Luigi received a letter telling him he had won the mansion Mario had gone there early and been captured by King Boo! It was a trick! After talking to Luigi, Professor Gadd found out what was going on and decided it was time to pass the Poltergust 3000 down to Luigi! With Professor E. Gadd's help Luigi saved Mario from King Boo and destroyed the mansion!
In Super Mario Sunshine, Mario's helpful companion device, F.L.U.D.D. bears a logo featuring Professor Gadd's face and claiming it was created by 'Gadd science incorporated'. Similarly the magic paintbrush which Bowser Jr. uses to wreak havoc on Isle Delfino also bears the same logo and Bowser Jr. says a strange old man in a white coat gave it too him, a description which F.L.U.D.D. seems to recognize, this is almost certainly Professor Gadd.
In Mario Party 6, he has his very own board called E. Gadd's Garage. In this board, he helps the players by using his latest inventions during the day and night and of course, buying stars for 20 coins when you reach the star space.
In Mario & Luigi: Superstar Saga it appeared Professor Gadd had opened a new cafe in the Beanbean Kingdom and every time Mario & Luigi concocted a new coffee blend in the cafe Gadd arrived to taste it (claiming that he can smell it from all that distance) and gave Mario & Luigi some kind of special item as well.
In Mario & Luigi: Partners In Time, the Professor was the local brain and gave useful tips to Mario and Luigi. However Gadd did not actually appear in person in this game and used his inventions during the game like the Hydrogush to wash away the Shroobs' foul deed, Stuffwell and the Time Machine.