From the Super Mario Wiki
Dr. Ryuta Kawashima (Japanese: 川島 隆太) is a Japanese neuroscientist and host of the Brain Age franchise. In the Brain Age games, Kawashima is represented by a polygonal version of his own face, similar to Andross.
Dr. Kawashima appears as an Assist Trophy in Super Smash Bros. for Nintendo 3DS / Wii U, using his Brain Age appearance. When summoned, he immediately disappears, but causes numbers to fly across the screen. If two numbers collide, they add into a larger number, and if the two numbers equal ten or greater then they explode. The explosion is bigger and lasts longer if the numbers equal exactly ten. Players can also attack the numbers to knock them away.
Dr. Kawashima reappears in his disembodied form as a Mystery Mushroom costume in Super Mario Maker. It can be unlocked after clearing the Event Course "Dr Kawashima's Athletic Training", which was released on December 20, 2016.
Super Smash Bros. for Nintendo 3DS / Wii U Trophy information