Ryuta Kawashima (川島 隆太, Kawashima Ryuta) is a real-world neuroscientist from Chiba City, Japan, who is known for his studies on the mapping of the various regions of the human brain and the effects of age on one's ability to learn. His most well-known work is a book entitled Train Your Brain: 60 Days to a Better Brain, which claimed that continued activities that stimulate the brain, such as basic mathematics and reading aloud, could be used to stave off the mental effects of aging. The book became a success, particularly in Japan, and his work was later adapted as a video game series by Nintendo, entitled Brain Age. In the games, Dr. Kawashima appears as a polygonal head, acting as a trainer for the player.
Dr. Kawashima is mentioned by name in the PAL version of Brawl in the title of the music track "Dr Kawashima's Brain Training: How Old Is Your Brain?", referred to as "Brain Age: Train Your Brain in Minutes a Day" in the NTSC version. This is to maintain consistency with the PAL region title of the Brain Age series, in which Dr. Kawashima is named.
As an Assist Trophy
Dr. Kawashima appears in both versions as an Assist Trophy. When released, he floats into the background. Once he is settled, large numbers from 1 to 9 will fly vertically and horizontally across the screen at random. The numbers do not hurt fighters but can deflected 90 degrees by their attacks. If two numbers collide, they will be added together; if the result is 10 or more, the new number will then explode after a short delay, with a significantly larger explosion similar to the Smart Bomb if the result is 10 exactly. Uncommonly, all the numbers produced will be numbers that add up to 10, like 6 and 4 or 3 and 7, guaranteeing sums of 10.
As an Assist Trophy
Dr. Kawashima reprises his Assist Trophy role from previous games. He seems to function the same as in Smash 4. One notable change is that his appearance changes the background of the stage to a white graphed background. He cannot appear in Boxing Ring, Coliseum, Frigate Orpheon, Green Greens, King of Fighters Stadium, Kongo Falls, Midgar, Moray Towers, New Pork City, Princess Peach's Castle, Prism Tower, Tortimer Island, Town and City, and WarioWare, Inc.