Osamu Kasai is a minor character in Battle Royale II: Requiem. He was a member of the school's Literature Club and transferred to Shikanotoride Middle School in an attempt to improve his grades, though the transfer did not achieve that goal. Among his classmates he was generally well liked, but he came across as timid and shy, and he harbored a romantic interest in one of his fellow students.

As a student of Class 3-B, Osamu is drawn into the BRII program alongside the rest of his class and takes part in the armed mission against the Wild Seven. He survives the initial assault on the island and the later standoff at the rebels' fort. In the final phase of the story, after the surviving students are given a chance to escape to safety, Osamu chooses to return to the battlefield, accompanying his classmates Takuma Aoi and Masami Shibaki in the final battle. This decision stands in contrast to his earlier withdrawn and hesitant nature, showing a quiet willingness to stand with his friends when it matters most, although the film does not explore his inner thoughts or the reasons behind his choice in any depth.

His key relationships are with his fellow students, particularly Takuma Aoi and Masami Shibaki, with whom he fights during the closing battle. His crush on a classmate reveals a softer, more personal side to his character that is otherwise overshadowed by the surrounding violence.

In terms of notable abilities, Osamu is not depicted as having any special combat skill or unique talent. Like the other students, he is given the standard-issue rifle and equipment used in the mission, and his significance in the story comes from his presence and his decision to join the final fight rather than from any distinctive prowess.