Satoshi Oosugi is Saki Morimi's university classmate and close friend, joining her on a graduation trip to New York. He carries persistent, unreciprocated romantic feelings for Saki, fueling jealousy towards Akira Takizawa, whom he views as a rival. Oosugi initially distrusts Takizawa, actively attempting to denounce his character.

As part of the Eden of the East group, Oosugi helps develop and utilize their image recognition system for identifying people and objects via social networking. Despite personal reservations, he assists the group's efforts supporting Takizawa during investigations into the Seleção conspiracy. In the second film, he aids Saki in locating Takizawa's mother, using the Eden system to trace a dog's ownership. This leads them to a Thai restaurant where they question a woman named Aya.

At the restaurant, Oosugi discovers a photograph showing Aya with former Prime Minister Iijima at an amusement park roughly twenty years prior, suggesting Takizawa might be Iijima's illegitimate child. When secret police arrive under Mononobe's orders, Oosugi deliberately delays them, enabling Saki and Aya to escape through a back exit. This self-sacrifice gives Saki the chance to relay information about Takizawa's origins. Oosugi later urges Saki to meet Takizawa directly to share her findings, prioritizing her connection over his own feelings.

Oosugi stands approximately 1.90 meters tall with brown hair and grey eyes. His character evolves from possessiveness and jealousy towards loyalty and selflessness within the group. He also experiences comedic misfortunes, including being stood up for lunch plans with Saki and becoming intoxicated to the point of vulnerability.