This page serves as a dump for the many, many Skull Face headcanons I have living in my brain at all times. This page will probably always be a big work in progress, also they're grouped by era/theme so I can keep track of them a bit better.
Earlier Life (up to roughly 1957)
He had a number of run-ins with the Soviet secret police in Hungary
He was involved in the 1956 revolution, fleeing Hungary soon after
Colonel Volgin's involvement in quashing the revolution gave Skull Face a particular hatred of him later in Tselinoyarsk
The parasites he used in his assassinations were supplied by a branch of the Philosophers, who found him a useful tool to get at the Soviets
The Philosophers acted as his benefactors in lieu of him having a family to take care of him in his childhood and teenage years
Nihilistic and somewhat overconfident in his youth, his inability to feel pain left him with very little to fear, driving him to reckless, often self-destructive behaviour, taking on bigger risks with each assassination he carried out
Cipher Era (1957 - around 1974)
Upon making him his XO, Zero sought to provide Skull Face with a more suiting wardrobe, purchasing him a fine suit for him as the first of many gifts to him
Skull Face also received an expensive record player from Zero during their time in America, in addition to inheriting Zero's collection of old records (mostly of classical music)
Skull Face became particularly attached to a record of classical music composed by the Hungarian Franz Liszt, especially Liebestraum No. 3, which he played almost constantly at home
He regularly keeps up to date with European affairs while in America
He finally gave in to Zero's badgering about taking him to see a film, it's there he first met Para-Medic. She found him fascinating (in a sort of movie monster sort of way)
He makes use of Cipher's resources to relearn Hungarian through self-study, ignoring Zero's not-so-subtle discouragement from using anything but English
When he first joins the SAS and meets Zero, his English is relatively poor, speaking with an unidentifiable accent that's a mix of all the languages he's picked up in his youth
Once given significant power at Cipher, he quickly becomes his sneaky, backstabbing self from his spy days. He makes a habit of snooping when Zero's back is turned, finding out the truth about himself and the parasite therapy searching through old records inherited from the Philosophers
It's in these same records he stumbles upon information on the vocal cord parasites, and begins formulating his plans
Later Life (1974 - 1984)
He had a decent amount of respect for Strangelove, and was particularly disgusted at Huey for killing her so indiscriminately
General
He is a fan of Sergio Leone's films, particularly For A Few Dollars More and The Good, The Bad and The Ugly
He wears so many layers partially as a way to hide, and partially as a way to protect his skin from the sun
His music tastes are very moody, with a trend towards folk and rock
He tends to default to cursing in Hungarian, finding English curses much less effective
The parasites mean he doesn't need to eat or drink to survive, and he tends to go without either of these. He still drinks whiskey from time to time (with Zero) in addition to black coffee
He never saw The Boss as an all important martyr like Zero and Big Boss did, but instead saw her as just another victim of the cycle of violence in the world, like himself and his people