Unlike some other synths, Delta has always known who - or, more accurately, what - he is. He was built by the Institute, a secretive scientific organization based in the Commonwealth. Unlike some other gen-three synths, Delta (or "D1-T8", as he was known then) was never sent to the surface. He worked in the Institute's BioScience division, responsible for furthering its understanding of bio-engineering, medicine, crop productions and pharmaceuticals. D1-T8 was present when another employee, Dr. Virgil, sabotaged a project to introduce the FEV virus to human lab subjects. Post-mortem interviews saw no cause for concern, but "Incident V" was last in a long line of occurrences which left D1-T8 questioning the Institute's mission. Synth-human cooperation was possible, he knew, but never at the cost of innocent lives.
D1-T8 eventually left the Institute with the help of the Railroad, though not without significant risk to himself and his rescuers. Under the name "Delta", he was relocated to the town of Goodneighbor, where he began treating patients using the medical expertise he had picked up in the BioScience division. Due to his relationship with the Railroad, Delta's clinic became a halfway house for synth refugees. He later ended up sheltering ex-Brotherhood Paladin Danse after the latter was revealed to be a synth sleeper agent.
Delta's competent medical expertise and calm bedside manner have made him popular throughout the Commonwealth, and his skills are often sought by those living outside the city. Patient and gentle, he is nonetheless willing to stand up for what he believes in, even if that means butting heads with the mayor of Goodneighbor over Delta's decision to treat patients in Diamond City. However, for his faith that synths and humans can eventually coexist, it's unlikely that anybody outside the Railroad knows of Delta's true nature. He may live and work amongst humans, but synths are no more kindly looked upon in Goodneighbor than any other part of the Commonwealth. Were the Institute to learn of his whereabouts, Delta isn't sure he could count on the people of the wasteland to protect him.