Left was the younger adoptive brother of Delta (Brother). In 1920, he was found dead when Delta was sixteen.
Left was raised with an older adoptive brother, Delta, and the (presumably biological) son of the couple of raised them. Since the transporter was located in Germany when Delta arrived, there is a high probability that Left lived in Germany or Europe with Delta.
When his brother was 16, Left died. Despite his body being covered in bruises, the police did only a brief investigation and ruled his death a suicide. Later, Delta found out that the killer had paid the police a massive bribe to keep the truth hidden from the public. Left's murderer is a mystery.
The injustice of his death led a sorrowful and resentful Delta to create Free the Soul in order to purify humanity into a higher species. Delta decided that Left's body would be the basis of this higher species. Delta desired to create a utopia in which every single human looked like Left with the exact same name and face, and everyone has the same beliefs, thoughts, opinions and values with no sense of individuality, since differences lead to conflict, and subsequently hate, violence, discrimination, etc.
After Free the Soul was created, Delta kidnapped a group of scientists to create clones of Left using Left's DNA and artificial uteruses. As of 2074, four generations of clones have been made.
Virtue's Last Reward
In Dio END, Sigma Klim, Phi, and Dio escaped Rhizome 9. Phi hunted down Dio, who had been trying to contact the Myrmidons. After some questioning, Dio revealed Delta's (Brother's) and Left's background among other things.
Dio was revealed to be a 4th-generation clone of Left, and as he was the best of his kind, he became the Myrmidon leader. It can be presumed that Left's face looked very much like Dio's. What changed between the generations of Myrmidons is unspecified.
Zero Time Dilemma
Left is mentioned in C-END: 1. Delta reveals that Left wasn't his biological brother, but rather, his adoptive brother.