Quirinus Quirrell is a British wizard who finds one of Tom Riddle's horcruxes around March 1991. Tom Riddle then takes possession of Quirrell's body and assumes his identity to become a teacher at Hogwarts.
In June 1992, in a graveyard a few miles from Hogwarts and Hogsmeade (canon), Voldemort left Quirrell's body. Relieved beyond comparison, Quirrell could only yell "Free!… Free!!!" before Voldemort ruthlessly killed him in the process of making a horcrux for Hermione Granger.
Quirinus was born in 1955 and was Sorted into Ravenclaw, being something of an academic overachiever. Tom Riddle may have slipped or been unaware of that backstory, as he claims to have been in Slytherin.
- Ch. 79, Rufus Scrimgeour reads a file on Quirrel: "Born the 26th of September, 1955, to Quondia Quirrell, of an acknowledged tryst with Lirinus Lumblung... [...] Sorted into Ravenclaw... O.W.L.S. quite good... N.E.W.T.S. in Charms, Transfiguration... an Outstanding in Muggle Studies, impressive... Ancient Runes, and ah yes, Defense. An Outstanding in that as well."
- Ch. 19, Riddle as Quirrell says "[...] to the one I held as a young Slytherin".