As a real person
Dr. Kawashima was born on May 23, 1959. He studies the brain at Kawashima Laboratories. In the 1970s, he enrolled at Tohoku University. He has written various books on his studies, and is very well known in Japan.
In the Brain Age series, Dr. Kawashima is portrayed as a cel-shaded, polygonal floating head. His hair and thick eyebrows are a black color (natural for Japanese people), and he wears oval-shaped glasses.
In later games, it is shown he has floating hands that usually stay off-screen, but hover to his side when he holds objects, or makes hand gestures. In Brain Age: Concentration Training, it is seen that his head can open up, and contains captions (such as "Brain") that float out.
Brain Age series
Dr. Kawashima has partnered with Nintendo to develop Brain Age: Train Your Brain in Minutes a Day! in 2005. The player could track their "Brain Age", the younger the age, the better the score is. Players participate in various engaging activities to train their brains, usually consisting of basic mathematics and memory exercises. The game also featured 100 Sudoku puzzles.
In 2007, Brain Age 2: More Training in Minutes a Day! was released in North America. Many of the training exercises from the original game returned, with many new puzzles to play.
In 2009, Nintendo released the Nintendo DSi. On the DSiWare service, the "Express" series (known in the PAL region as "A Little Bit of..." series) launched, which included small downloadable versions of games seen on the Nintendo DS. Brain Age was one of the franchises represented. Brain Age Express: Arts & Letters, Brain Age Express: Math, Brain Age Express: Literature, and Brain Age Express: Sudoku were released all together. All versions included a Brain Age Check, based around the subject of the title, and Brain Training with the specialized topic. Brain Age Express: Arts & Letters, and Brain Age Express: Math were built-in software for the Nintendo DSi XL (along with Flipnote Studio and Photo Clock, both previously downloadable.)
In 2013, Nintendo released Brain Age: Concentration Training for Nintendo 3DS. This time, Dr. Kawashima has voice acting, and can become a demon to significantly increase the game's difficulty. In Brain Training, Dr. Kawashima is in a form similar to in past games, where his speech appears in text (with the same "Bebebebebe" sound).
Dr. Kawashima made his customary appearance in Dr. Kawashima's Brain Training for Nintendo Switch, guiding the player through puzzles in the usual fashion.
Super Smash Bros. series
In Super Smash Bros. for Nintendo 3DS/Wii U and Super Smash Bros Ultimate, Dr. Kawashima appears as an Assist Trophy. When he is summoned, he causes numbers to move across the screen and can be hit by players. The background turns into a black and white grid as well. When numbers collide, they are added together, and they explode if the sum is 10 or more. The explosion is bigger if the sum is exactly 10.