Toshiaki is over-achieving and mega-competitive. He is 12-years old. He delights in beating Victor at his own game and like a power-hungry mad scientist, Toshiaki will stop at nothing to win the top prize in the school’s science fair—even if it means stealing Victor’s ideas to do it.
In the film, when Toshiaki revives his deceased pet turtle, Shelley, it mutates and grows to a horrific size. In this state, Shelley closely resembles popular Japanese Kaiju monster Gamera. Additionally, the scene in which Shelley crushes a police car with its foot is a reference to the teaser trailer for the 1998 Godzilla movie, an American reboot of the popular Japanese Kaiju franchise. All of this is given further significance by the fact that Toshiaki is Japanese.
Toshiaki can be interpreted as a dark twist on the various child characters of Godzilla and Gamera films from the late 60s and early 70s, who often had a fondness for monsters and a deeper understanding of them compared to the adults. Gamera himself was billed as the "Friend of All Children" and protected them from other kaiju, so Shelley turning on Toshiaki is a possible subversion of that relationship.
Toshiaki is a 12-year old boy. He is younger than Nassor.