Toshiaki is the tertiary antagonist of Frankenweenie. He is one of Victor Frankenstein's arch enemies and pitcher of the little league team at their New Holland Elementary School. He is a 12-year-old boy with Asian features and accent, which makes it possible that he is a foreign exchange student. Toshiaki is an insecure bully. He wants to be the best at everything; so in spite of the popularity his position on the school baseball team provide him, he is extremely jelaous of Victor's brilliance. In site of his dislike for Victor, however, Toshiaki seems to like Victor's dog named Sparky. His desperation to be the best can reach a dangerous level, usually to other people rather than himself. This suggests that Toshiaki may be an egomaniac and or megalomaniac.
He is first in Victors science class, sitting near the back with his fat assistant/friend Bob. After their substitute science teacher explains the mechanics of someone getting struck by lightning, he assures the class that such an event is vary uncommon. However, Toshiaki and Bob contradict him on that. Toshiaki states; "There is a lightning storm almost every night.". This interruption makes a shambles of the lecture, as it encourages Nassor and Weird Girl to start arguing over the cause of all the storms. At the end of the class, Toshiaki and Bob sign up for the science fair as partners.
Toshiaki is later seen at a little league baseball practice. The coach, Mr. Frankenstein, finally convinced his son Victor to participate on the team. Toshiaki and the team catcher, Nassor, rather liked the idea, as gave them a chance to beat their least favorite classmate. Toshiaki score to strikes on Victor respectively, and was about to score a third when (to his and Nassor's dismay) Victor hits a home run. The practice was ended however, due to the fact that Victor's dog Sparky chased the ball into the street and was killed by a car.
Toshiaki and his science fair partner Bob are later seen in Toshiaki's garage, discussing their science fair project, when another science fair student (a hunchback student named Edgar E Gore) shows up and proves to them that he has an invisible goldfish. When Edgar refuses to tell how he accomplished this miracle, Bob (on a facial cue from Toshiaki) kicks (or rather pushes) Edgar out of the garage while claiming that his and Toshiaki project is "way better". When Edgar asks what said project is, Toshiaki says that it's a secret before closing the garage door on him. Bob then scraps their previous idea (Sea Monkeys) while Toshiaki tries to think of way to best Edgar.
The next day, Toshiaki puts his insane idea into affect. He and Bob build a jet pack fueled by a carbonated beverage. In spite of his protests, Toshiaki forces Bob to be the guinea pig, and launches him from the roof while he films it. The jet pack supports Bob's considerable weight for about two seconds before running out of fuel and drops him to the ground were he breaks his arm. As Bob is loaded onto an ambulance, his grandmother angrily asks him and Toshiaki (who is still filming for some reason) what they were doing on the roof, to which the two boys reply; "Science". Though they didn't intend it, this lead to the science teacher being fired from his position.
Toshiaki is later seen in the science classroom, where he and the rest of the class are appalled to find that their dimwitted gym teacher is the new science teacher.
Toshiaki and Bob are later seen out on the bleachers lamenting over the fact that they'd never be able to beat Edgar, much less win the science fair. Nassor then arrives and tells them that Edgar had fooled them with a parlor trick. Edgar (who unfortunate enough to arrive at that precise moment) was accused of fraud by Toshiaki and Bob. Edgar angrily claimed that it was real (it had previously disappeared for unknown reasons) and that it was dead to. When they asked him how, he answered; "Victor did it! With lightning! And boom! And "sss". Just like he did with Sparky". Toshiaki intently asks; "He brought Sparky back to life?". When Edgar tried to take back what he said; Toshiaki, Bob, and a suddenly interested Nassor back him into a corner. Under the combined pressure of all three bullies, Edgar agrees to show them Victor's secret.
That night; Toshiaki, Edgar, Bob, Nassor, the Weird Girl (and her creepy cat Mr. Whiskers) go break into Victor's house after he and his parents left to find an escaped Sparky. They go up to the attic where Victor conducted his experiments. There, they get all the information they need to mimic Victors work. Instead of assisting Bob in his idea (a new and improved sea monkeys project) Toshiaki goes solo with hopes of resurrecting his beloved turtle Shelly.
Toshiaki and Nassor (who wants to resurrect his hamster, Colossus) go to the pet cemetery. Toshiaki digs up Shelley's body and goes back to his yard. There, he constructs a lightning rod, and hooks it up to Shelley's body. However, there were two problems. 1. The process affects each monster differently. 2. A box of "Miracle Gro" right next to Shelley. These two variables caused Shelly to come to life as a giant turtle monster.
Toshiaki ran to Victor, from whom Bob was already asking help with his sea monkeys, who had also gone haywire. The three boys watch as both the sea monkeys and Shelly head for the Dutch Day Festival. Victor and Bob follow, but Toshiaki remained behind long enough to activate his camera. Then, with a wicked snigger, he heads in the direction of the festival.
By the time Toshiaki reaches the science fair; Shelley, and all the monsters his fellow troublemakers unleashed, were on the rampage. Nassor and his mummy hamster arrive, at which point he orders Colossus to kill Shelly. Toshiaki watches with amusement as Shelly crushes the undead rodent with his foot.
In his last seen; Toshiaki has secured a high elevation on the fares wheel from which he films the progress of Shelley's rampage. But Shelly (who clearly doesn't recognize Toshiaki) grabs him up in his jaws, bringing the documentary to a screeching haunt. From his even higher elevation, Toshiaki screams to Victor for help. Victor is able to save Toshiaki by electrocuting the wet ground on which Shelly stands. In "shock", Shelly drop Toshiaki to the ground. Shelley then dies and turns back into his ordinary corpse. Toshiaki then laments over his once again dead turtle. What ultimately happened to him and Shelley is unknown.