A nephew to Kelanis Guildmaster Abraham Kachinsky, Milo was heir to a sizeable percentage of his family's shipping lines. His older brother, Victor Kachinsky, led a guerilla squad of rippers in surgical strikes against the Confederacy on Artesia Prime; in fact, Victor's reputation for using spider mines to soften up Confederate forces before an ambush earned him the nickname "Creeper Kachinsky." Milo worshipped his brother and spent long hours learning how the war hero had customized spider mines to achieve his military goals.
Young Kachinsky had a knack for robotics and was expelled from upper school for "remotely automating" the door to the women's locker room. He later learned to buckle down, inspired by his brother's achievements in the military (and hoping to receive similar decorations for his own work). Milo excelled in his classes, skipped several grades, and soon received a degree in mechanical engineering from Sakai University at the age of 17. He and Victor had plans to combine their skills in an ambitious new endeavor: the formation of an independent engineering corporation that would design starships with more advanced AI navigation than anything currently being built.
Two months after Milo's graduation, Victor was killed in his sleep by Dominion ghosts in an attempt by Emperor Mengsk to "simplify" relations between the Combine and his new government. Some theorize that Mengsk wanted the Kachinsky starship blueprints as well; the studio where Milo and Victor had been designing their prototype was ransacked.
The Kel-Morian response was angry but reserved; the Combine was in no place to mount any sort of military retaliation. After his uncle Abraham made a speech about working toward peace with the Dominion, Milo decided that he could no longer be a part of the Kel-Morian effort - or his family. He left Moria and sought out Jim Raynor. Victor claimed to have met Raynor in a bar on Mar Sara once, and he had spoken highly of the marshal-turned-rebel. That was enough for Milo, who wanted to join the fight against the Dominion - and wanted to follow a hero like his brother.
The few members of the Hyperion's crew who know of his past give Milo Kachinsky credit for his decision to strike back at Mengsk and abandon an easy life of familial wealth. Rory Swann - a vicious critic of the Kel-Morians - originally argued with Raynor over bringing Kachinsky on board. Swann has since grown to respect the idealistic young engineer, even putting him in charge of the robotics wing of the Hyperion's weapons research.