Not all of the above is precisely canon, as William Gibson, who created the character, was careful not to give away exactly who he really was. There were hints that he may have been Kent Allard, an aviator in the first World War, and simply faked his death after the war by plane crash. There were also hints, because The Shadow was so fond of assuming false identities, that Kent Allard really did die and he was pretending to be him, resurrected from the dead, although my take is that the original Kent Allard was an early alias.
There was also an allusion, once, to an unnamed soldier or spy in World War One who suffered a disfiguring facial injury, and who might be his true identity. The truth is that a lot of soldiers really did, which seems important to remember on this of all days. The details were never specified, but in a few stories there would be a criminal who, for some reason or another, glimpsed The Shadow without disguise just before their death. The sight tended to leave them frozen in horror, was only described as skull-like, and one man responded that he saw The Shadow was 'a man of many faces, with no face of his own.'
Obviously I've had to draw from these bits of the novels and make an interpretation.