This world is another result of Devilman-ViolenceJack shift (contains unmarked spoilers from both mangas)The moment Ken and Ryo were first seen fans immiediately nicknamed them "Violence Jack and Satan", and their code names are names of Amon - demon which Akira fuses in Devilman - and Lucifer - Satan, main antagonist of Devilman, who hides himself as Ryo Asuka. After the end of Devilman, God (or Satan, I heard two versions) hit Reset Button, creating world of Violence Jack. Satan was reborn as three different people, so was Devilman and Violence Jack was one of his reincarnations. After the event of the series Reset Button was hit again, creating Devilman Lady and then again, creating Shin Violence Jack. It may be possible that different versions of Mazinger Z (or even all Go Nagai manga, as Violence Jack featured number of characters from his different works) may also exist in this continuity and world is constantly resetting itself, shifting between different versions over and over again. The result of latest reset was a postapocalyptic world in which Mazinkaiser got mixed with Devilman, creating Mazinkaiser SKL and Ken and Ryo are reincarnations of Violence Jack and Satan.
This is a distant sequel to Shin MazingerBefore the war mentioned in the series there was war with Mycean race, which was freed at the end of Shin Mazinger, during which Mazinkaiser SKL was created. General from episode 3 really is Boss.
Both Ken and Ryo are clones of KojiThe two of them are a probably the only survivors or just prototypes for a sort of mass-produced Mazinger Corps with all clones based on Koji Kabuto himself how ever to avoid any and all plausible issues of morality these two were made into psychopaths possibly by crossing brainwave data of Ryoma Nagare and Hyato Jin for the newly created Mazinkaiser SKL there could be an entire army of tweaked more sane but less "effective" Koji clones waiting in mass made Mazinger's or even Shin Mazinger's that are used when the situation does not call for "Kaiser" orr.......
The heroes are actually son's or at least one of them is a son of Koji and the other may be a cloneLikely Ryo who got seriously messed up in the head after seeing his father die in the previous war wanting to protect the world or get rid of his pent up agression he teamed up with a clone earned the Kaiser and have been hell ever since