Young Professor Elvin Gadd
From the Super Mario Wiki
“Oh, criminy, I have guests! I was so caught up in my research I didn't notice you.”
Young Professor Elvin Gadd is the younger version of Professor Elvin Gadd who appears in Mario & Luigi: Partners in Time, identical in appearance to his older self with the exception of his hair being black. Young E. Gadd is encountered in his lab on Thwomp Volcano as it is erupting, studying Thwomps and the history of the volcano itself. During the eruption, Young E. Gadd's lab is destroyed by Thwomps that fall through the building, including a massive, flaming one that is about to explode. Seeing Baby Mario douse the giant Thwomp, the memories of his present self suddenly start to be rearranged, and he has the idea of building the Hydrogush 4000.
Afterward, young E. Gadd mentions that he plans on relocating to an affordable lab at the edge of Boo Woods for "paranormal research", a reference to Luigi's Mansion, and the young version is never seen in that game again.