Before Shiro became the Black Paladin, he was known as a pilot of legendary reputation at the Galaxy Garrison and was the pilot for the Kerberos Mission under Commander Samuel Holt along with Sam's son, Matthew Holt. The mission's purpose was the study of ice samples for signs of extraterrestrial life. However, Shiro and his crew members were captured by the Unnamed Galra Commander of the Galra Empire. Sam was sent with weaker prisoners to a work camp while Shiro and Matt were sent to fight for Zarkon's entertainment in the gladiator arena against Myzax.
Matt was intended to fight first, but Shiro knew the boy would not survive so he feigned a bloodthirsty desire to fight and stole the blade offered to Matt, minorly injuring him with it so the boy would not be subject to a brutal battle. Shiro fought Myzax instead and was victorious, claiming the title of "Champion". These events were witnessed by a fellow prisoner named Xi. The details of what transpired during Shiro's captivity under the Galra Empire afterward remain unknown.
During this time, Shiro somehow had his right arm replaced with a powerful prosthetic, and it is known Haggar intended for him to be the Empire's "greatest weapon". On Earth, the Galaxy Garrison announced the Kerberos crew had gone missing five months after leaving Earth, and presumed all crew was dead due to a pilot error that caused a deadly crash. One year after capture, Shiro was rescued by Ulaz while being examined by Galra technicians; Ulaz gave Shiro information about the Blue Lion's existence on Earth and coordinates implanted into his prosthetic arm that divulged the location of a base used by Blade of Marmora, but Shiro suffered a head injury from an explosion during escape, leaving him with amnesia as he fell into an escape pod destined for Earth.