Although he never made an appearance in Brawl, his name was mentioned in the PAL version in the music track Dr. Kawashima’s Brain Training: How Old is Your Brain.
Dr. Kawashima makes his debut as an Assist Trophy in the game. Once he is summoned, he will fly off the screen leaving bubbles containing numbers from 9 thru 10 that float in from the blast lines. The numbers cannot hurt players, and players can redirect the numbers' trajectory into each other to add up two numbers at a time. Numbers can also vary in speed depending on the knockback given to them.
If the numbers combined are greater than 10, it will create a small explosion, similar to throwing most explosive items. However, if the numbers combined equal 10, it will create an explosion similar to a Smart Bomb, but the user, or team, that summoned him are not affected by any explosions
Dr. Ryuta Kawashima is the neuroscientist behind the Brain Age series. In this game, he sends numbers 1 to 9 flying onto the screen to crash into one another. If two colliding numbers add up to 10 or more, they'll explode. If the two numbers total 10 exactly, the explosion will be even more impressive!
- DS - Brain Age: Train Your Brain in Minutes a Day! (04/2006)
- DS - Brain Age 2: More Training in Minutes a Day! (08/2007)
Dr. Kawashima reprises his role as an assist trophy in Ultimate. He also is an unlockable spirit, and can be enhanced. Once enhanced, Kawashima will change into his devilish form from Brain Age: Concentration Training's Devilish Training mode.
- It is possible to make the numbers disappear like other Assist Trophies, without them having to disappear off-screen, by inflicting minor knockback (such as a jab) on a number, without it colliding with another number for a short period of time.
- Dr. Kawashima is the only assist trophy to not originate in a video game.