Violence Jack was an undercover cop who lost his memories because of the earthquake, just like Angel Cop and Mad Bull 34. Some reasons could be:
Ushio Mizuki, Jack's sidekick in Shin Violence Jack, is actually Tare Makimura, Miki's little brother from Devilman.When he sees Miki chained and with a mask, he grows terrified and runs away. When he finally stops, he realizes that he was about to remember something that was so frightening that he couldn't bring himself to remember it. What his memory is repressing is his horrible death at the end of Devilman, when the lynch mob decapitated him before they killed Miki. Likewise, when he runs back to help her ("even if it makes me remember"), he says to himself that he can't run away because otherwise he would go back to "being the cry-baby I was when I was little," which is an apt description of Tare in Devilman.
Violence Jack is Samurai Jack.
Jack is the reincarnation of Akira AND AmonFollow my logic here. Jack's personality seems like a combination of the two, and in addition, he has traits that wouldn't make sense if he was purely Akira or purely Amon. Akira evidence
- Satan is actually overjoyed and obbessed with him, which Neo Devilman and other media have established Satan cared about ONLY Akira Fudoh and couldn't give half a crap about Amon.
- He seems to care for Miki instinctively, another Akira thing
- His speech to Mondo during the El Dorado arc mirrors Akira's various "The Reason You Suck" Speech that he would give out to various demons.
- His interactions with children is much nicer and less murderous or simply annoyed as Amon's interactions with them in the OV As and Amon's manga.
- Looks like an adult, muscled Akira.
- Willingly gives his powers to those who need it twice
- Has Amon's markings from Demon Knight in Shin Violence Jack,
- Actually gives a crap about Siren
- His speech on Might Makes Right in the Slums arc
- His apparent knowledge on the goings on of The Skull King.