Born 37 BBY, Imperial Stormtrooper Captain Kurt Stein grew up in the same as orphanage as DW-418121 (Animal), although Stein had known nothing but the orphanage since he was 5 years old, found abandoned in the streets of Corellia, his drug addict mother dead besides him from a drug overdose. Growing up, he was bullied very harshly due to his orphan status. His future enrollment into the Empire's Sub-Adult Group would give him the family, support, and structure he never had and would allow him to learn to stand up for himself against bullies; It would be a deadly foreshadowing of the excellent Stormtrooper infantry officer he would later become, becoming totally devoted and fanatical to the Empire, the only family that ever cared about him.
Stein was Animal's senior at 16 years old when the 14 year-old future Imperial Master Sergeant was taken to the orphanage. Both men formed an unbreakable bond with another orphan known as Motley. Unlike Animal & Motley, Kurt was part of the Sub-Adult Group much longer and was already a highly respected Junior Officer Cadet in the program when Animal joined (Motley had arrived a year earlier). Stein would be a role model for both the younger Animal and Motley. Unfortunately, Motley would die several years later at the Academy of Carida, where all three men would be enrolled to become future Stormtroopers. Kurt was two years ahead of both his friends at the Academy of Carida and---enrolled a year earlier at 17 years old verses 18 years of age---already enrolled after academy graduation into the Officer's Candidate School after two years of outstanding performance with Death's Whisper Legion in heavy combat during the Western Reaches Pacification Operations in 15 BBY & 14 BBY, and wasn't present during the accident---which Animal took personally---but was interviewed during the investigations into Motley's death, because he had known both the deceased and Animal so well.
Upon graduation from the Officer's Candidate School in 13 BBY, Kurt's outstanding academy, exceptional field experience (which earned him a Crimson Star), and officer training landed Kurt into the legendary 501st Legion or Vader's Fist. He would rise up the ranks from Second Lieutenant to Captain with his time in the legion. Later, he would be assigned to Vader's own protection squad within the 501st and, several years later, assigned as one of Vader's personal Staff Officers assigned to the Dark Lord's personal security.
In 2 BBY, Vader had Kurt assigned to Joker Platoon to, on paper, replace the unit's deceased commanding officer. The infantry captain was also there to be Vader's eyes and ears and keep an eye on both Commodore Valentino Mazzanti and Doctor Van Halfin The Third.