The prisoner on the Death Star was a male Human Jedi. This Jedi had previously served both the Jedi Order and the Galactic Republic during the time of the Clone Wars. After surviving the war's conclusion, the Order's dissolution, and the ascension of the Galactic Empire, the Jedi found himself incarcerated on the original Death Star by the year 0 BBY.
Being a Force-sensitive Human, this individual was trained in the Force's ways by the Jedi Order while the Galactic Republic was nearing its end. He lived through the brutality of the Clone Wars and the Jedi Order's abolishment by the treacherous Supreme Chancellor Palpatine. Consequently, the Jedi went into hiding as the Galactic Empire began the Great Jedi Purge. Remaining concealed for almost two decades, the Empire finally captured and imprisoned the Jedi on their secret battle station, known as the Death Star.
Not long before the Battle of Yavin, the Jedi incited a rebellion among the prisoners of Detention Block 2180, which included members of the Rebel Alliance captured during an operation taking place on Naboo. This uprising led to prisoners breaking free from their cells and seizing weapons from one of the station's armories. As the prisoners dispersed throughout the station, the Jedi led a significant group toward Hangar 85G in an attempt to board a Sentinel-class landing craft, nearly succeeding with only moments to spare before takeoff. The Jedi leader ultimately revealed himself in a desperate dash toward the shuttle, but almost immediately, Imperial shock troopers belonging to the 501st Legion managed to destroy the craft.
The enraged Jedi, now wielding two green-bladed lightsabers, cut down every trooper who dared to stand in his way. After a fierce and bloody battle, he was ultimately shot down. Due to this security breach, the troopers of the 501st were reassigned to locate the stolen Death Star plans.
Despite its failure, the uprising inadvertently proved to be beneficial. Although the Jedi perished, some rebels managed to escape the prison, successfully smuggling the plans off the station and delivering them to the Rebel Alliance leadership. Shortly thereafter, the Death Star was destroyed by Luke Skywalker.