He is a imbecilic simpleton whose success was attributed solely to the powerful effects of the Arista and Plavsky Particles, which eventually resulted in the overwhelming popularity of Gunpla battles, instead of any effort or smart business tactics used. He has no respect for anyone and holds no love for the business which made him what he is today, seeing it as little more than an essential tool for him to live comfortably in riches. Despite the Arista and Plavsky Particles having immense potential in changing the world, he stays content with the wealth the technologies earned him, stating that changing the world is too much work. However, despite his dim wit, arrogance and laziness, he is unscrupulous and wouldn't hesitate to rely on underhanded tactics to achieve his goals.
Mashita was a petty thief in Arian. He had snuck into the treasure room of the palace and stole an Arista, and it reacted, transporting him to another world, Earth. He had completely lost his bearings but then he met Baker (who was cosplaying as Sayla Mass) and upon finding out what the Arista could do, and the basis of Plavsky Particles, they came up with the business of using Arista for Gunpla Battle and thus became a rich businessman.
His business, the PPSE had grown into a global company in very a short time due to popularity with Gunpla battles, gaining a monopoly on the manufacture and supply of both the Gundam Battle System and the Plavsky Particles it uses.
Upon seeing Reiji in the World Tournament, he was shaken in fear as he wondered, what was Reiji doing in this world. He speculated that Reiji as a slightly older version of the Prince of Arian. Not wanting the prince around for long, and thus risk exposing himself, he attempted time and again with underhanded tactics to kick Reiji and Sei out of the tournament.
Following the tampering of the Race by C. Reiji invades Mashita's Mansion for the boy suspects he had something to do with the interference. Upon seeing Reiji's Royal Gem, set on his bracelet, Mashita confirms his suspicions that Reiji is the crown prince of Arian, which apparently poses a threat to himself and the secret of Plavsky Particles. However Reiji has no recognition of Mashita from his own world.
Ironically as well, the Arista which brought Mashita immense wealth, was thought by Reiji as nothing more than a more good-looking common stone.
Mashita remained silent during the quarter and semi-finals, due to him believing that both Nils Nielsen and Aila Jyrkiäinen could defeat Reiji. However, his foresighting proved wrong, as both Sei and Reiji managed to defeat their opponents and advance up to the finals.
In order to make sure that Reiji would be defeated, Mashita came up with a different plan: he hired Nine Barthes to build an improved Embody System, disguised as a pair of dark glasses, and then forced Tatsuya Yuuki to wear them. This not only increased Yuuki's ferocity, but also allowed Mashita to exert his will into Yuuki through the Arista in his pocket watch, using the large underground Arista as a medium.
However, due to unforeseen circumstances in which both Reiji and Aila's Arista reacting to their thoughts as well, they managed to eventually snap Yuuki out of Mashita's control, causing this plan to fall apart. Yuuki, after recovering his bearings, finally revealed that the Chairman was responsible for his brainwashing, at which point Mashita screamed and held his pocket watch up, with the Arista in it reacting with the one underground and summoning forth a real-size A Baoa Qu replica inside the arena (and apparently physical, since it managed to destroy the ceiling of the arena in its appearance).
It is also revealed that it was he who turned the second Meijin into the monster people knew him to be, when he goes to meet Tatsuya Yuuki and tells him to win at all costs, fighting Reiji with full ferocity and showing no mercy towards his opponents (the fighting style of the second Meijin).
During the fighters' attempt to destroy the large Arista in the center part of the large A Baoa Qu, they were attacked by several Mocks, which Chairman Mashita pointed out in shock as the new product that were going to be announced in the same tournament. Once the large-sized Arista had been destroyed Mashita was seen glowing, being forced back to the Arian world, due to the destruction of the Arista that brought him to Earth. He shouted that he was a success on Earth and he couldn't believe what was happening, before being forced back to Arian, pulling along Baker as well, who had decided to go with him.
However, he and Baker found a happy ending nonetheless, as they were last seen selling Mocks on Arian.