Caster's True name is François Prelati, a close friend of Gilles De Rais during the era of Jeanne D'arc. He was a known magus, a cleric and a practitioner of black magic, he conducted various demon summoning rituals and alchemy, he owned Prelati's Spellbook which later became Gilles De Rais' noble phantasm when he was summoned in Caster Class. He was later executed for his crimes and became a heroic spirit.
Although his then body and mind died and became a heroic spirit he survived by changing bodies and dying numerous times. In current timeline of [Fate/strange fake], he possesses a female body and goes by the name of Francesca Prelati who summoned him in the 'True and False Holy Grail Wars' as a Caster Class servant using herself as a catalyst. While Francesca possesses all of her memories, Caster on the other hand does not have any knowledge of the events after his first death.