Nobody knows much about his past, but he knew one thing was for sure. How he ended up how he was in a perfect life was unknown to everyone. Normally, Myotismon are banned to the Dark Area immediately, but something about Mysteros let him live in the normal area along with his five destructive Digimon friends. Yes, all six of them together are probably more destructive than they appear to be, but it's mostly not even Mysteros who is the most violent (That honor goes to the tied Puffy and Gear). Mysteros would simply sit on the sidelines reading books while trying to help Jim with singing lessons only for Puffy and Gear to wreck the peace causing the four of them to get into a fight only to end up with Killer and Charming attempting to break up the fights only for a punch of kick to be thrown at them causing all six of them to get into one dust ball of a fight. While he, Jim, and Puffy are Virus Attribute, only he can get in deep enough to get any important information as while Puffy and Jim are tough enough to fight in the Dark Area, Mysteros can simply waltz on in without any need to fight as Myotismon is a respected Digimon in the area. There's just something about his past, however, that seems to have the villains after him the most... Perhaps he is hiding something?
After a more famous Myotismon got destroyed yet again, his data got reborn into a Zurumon, which would end up becoming Mysteros. However, he had neither any memories of his old self or even the same intentions at all. While the true, evil Myotismon's memories and behavior became re-incarnated elsewhere, only to be destroyed by Rainboramon before it got any next chance to do anything. Years later, Grandis sent several Digimon to the new universe he created after the demise of ZeroDragoramon's physical form, Mysteros was one of those Digimon. He was a mere Devimon at this point, and already he had gotten hate simply for being what he was, even though he didn't act evil or anything bad at all. Some time passes and he digivolves into his default form Myotismon, and gets even worse treatment than before, this time with a Seraphimon who's out to get him. Outmatched by the Mega Level Digimon, he was nearly killed until Seraphimon got in the way of one of Gear's schemes, which, while it didn't kill Seraphimon to the point it would anger the Celestial Digimon, or in a worse case scenario the Royal Knights, when Seraphimon was about to attack again Gear stood in the way and blocked Serphimon's attack, soon joined by Gear's friends Puffy, Killer, Charming, and Jim. The five Ultimate Level Digimon for some reason didn't want Mysteros killed at all, the five explaining to Seraphimon that they've seen Mysteros before and know he isn't anywhere near as evil as Seraphimon thinks (Which in reality was merely a bluff by Charming). Unconvinced, Seraphimon demands proof of this kindness that Mysteros has and gives him one week to prove himself, and so he leaves and Mysteros befriends the group of Digimon who saved him, Charming saying that if they even dare have a Virus Digimon in the Royal Knights, it would make no difference as to why any Virus Digimon could be living an equal life as long as they weren't evil. This soon became Mysteros's own ideals , and tried helping out around and soon his evil reputation was gone, and Seraphimon said he was free to go when he returned 7 days later.
Zurumon is Mysteros's Fresh form that's seen mostly when degenerating from MaloMyotismon.
When Eaglemon attacked, Mysteros was degenerated into Pagumon. Not seen again as normally Mysteros warp digivolves right to Myotismon when his energy recovers from being a Zurumon.
When a Monochromon attacked, Mysteros digivolved into DemiDevimon to help stop Monochromon. Only appears for a brief while in the series as Mysteros is either Zurumon, Myotismon, or MaloMyotismon the entire series.
As a Devimon, Mysteros was given a rough past in life, but now makes use of his abilities as Devimon for more good intent now. Due to how much Mysteros hates his past, he is never seen as Devimon again once he digivolves back into Myotismon.
During Myotismon's quest to digivolve to Mega, he has to do the one thing he vowed to never do; Commit an evil act. However, that hopeless to prevent when Barbamon possessed him for a short while, and when he did digivolve, he didn't digivolve to his true Mega Form MaloMyotismon, but rather VenomMyotismon, and when Seraphimon shows up yet again, Myotismon's evil act is accomplished by destroying his former "enemy", which ends up breaking the spell Barbamon had on him and causing him to Slide Digivolve to MaloMyotismon and destroy Barbamon. Seraphimon, in reality, had survived the attack and knew all along that he was possessed by Barbamon, as both he and Mysteros knew that the Demon Lords were beginning to see the DigiDestined as a threat now.
Mysteros's true Mega form. In a way, his digivolution to VenomMyotismon was more or less a Dark Digivolution, where his digivolution to MaloMyotismon was more normal like the others were. Typically MaloMyotismon packed more power than most of his comrades Mega forms, only rivaled by BlackWarGreymon in power.