Michael “Mickey” Crespo was born on Luna in 2530 to insurrectionist parents. When he was young, Mickey’s parents died attempting to carry out a terrorist bombing, sending him into foster care. When he turned 18, Mickey enlisted in the UNSC as required to earn his freedom from foster care. He eventually joined the ODSTs, forming up with Alpha-Nine under Gunnery Sergeant Edward Buck just before the start of the Battle for Earth in 2552. Mickey and all of Alpha-Nine survived the battle and the war, partly due to luck and partly due to their exceptional skills.
Because of their skills, Buck, and eventually the rest of Alpha-Nine were recruited for the Spartan-IV program, though Dutch and the Rookie did not join since the former wanted more time with his wife and the latter died during the Draco III Rebellion. Following completing Spartan-IV training and augmentations, Mickey, Buck, and Romeo formed the new Alpha-Nine, and began service as Spartan super-soldiers. It was around this time, however, that Mickey was recruited by the United Rebel Front, using his familial connections to the insurrection and his dislike for fighting other humans to turn him into a traitor. Only a year after entering service as a Spartan-IV, Mickey, Buck, and Romeo were sent on Operation: COALPEPPER, where Mickey betrayed his squadmates and attempted to kidnap both Buck and Romeo for the URF. The kidnapping was a failure, however, and Mickey was detained by Buck and ultimately thrown in a Spartan holding cell under guard in the Spartan training facility and under the watch of training lead Jun-A266.
Mickey remained locked away in Spartan prison for over three years until after the Raising of the Guardians at the end of 2558. With Cortana taking control of the entire known galaxy, Veronica Dare gave Buck a mission to reunite all of Alpha-Nine and investigate a insurrectionist colony for technology that could be helpful to the fight against the Created. Though the mission ended in failure and the colony was made subservient to Cortana, Alpha-Nine remained reunited under Buck including Mickey and Dutch.