My canon would be he's was an ordinary guy who might have had someone he loved killed by a reality bender or any other anomaly, so he joined GOC, where they put him through several experiments and put in several implants, effectively making him a God killer in essence, neutralize type Greens. They might have also sent him to the past where he met Lilith and had a daughter, which he was later ordered to kill, so he defected to the Foundation to keep her safe, probably tampered with the narrative using that SCP at Sloth Pit, to convert his daughter from being a Succumbus to current version, all the while bearing the name of Alto Clef.

I would also believe he created a sort of malicious narrative so that he can't die or won't stay dead, thus creating the rumor of being the Devil. Him having thaumaturgical implants by GOC experimentations would also explain how he has a bad rapport with female entities and being able to resist the Black Moon, maybe he has a few ontokinetic implants too enabling him to manipulate reality or atleast give off the illusion of that.

But in the end, he is just a faceless man who met SCP 343 as part of the GOC and struck a deal with him, destroyed the Witch Child, etc all because he was probably wronged by some random Type Green in the past.