The origin of St. Valentine, and how many St. Valentines there were, remains a mystery. One opinion is that he was a Roman martyred for refusing to give up his Christian faith. Other historians hold that St. Valentine was a temple priest jailed for defiance during the reign of Claudius. Whoever he was, Valentine really existed because archaeologists have unearthed a Roman catacomb and an ancient church dedicated to Saint Valentine. In 496 AD Pope Gelasius marked February 14th as a celebration in honor of his martyrdom.
The first representation of Saint Valentine appeared in a The Nuremberg Chronicle, a great illustrated book printed in 1493. [Additional evidence that Valentine was a real person: archaeologists have unearthed a Roman catacomb and an ancient church dedicated to Saint Valentine.] Alongside a woodcut portrait of him, text states that Valentinus was a Roman priest martyred during the reign of Claudius the Goth [Claudius II]. Since he was caught marrying Christian couples and aiding any Christians who were being persecuted under Emperor Claudius in Rome [when helping them was considered a crime], Valentinus was arrested and imprisoned. Claudius took a liking to this prisoner -- until Valentinus made a strategic error: he tried to convert the Emperor -- whereupon this priest was condemned to death. He was beaten with clubs and stoned; when that didn't do it, he was beheaded outside the Flaminian Gate [circa 269].
Saints are not supposed to rest in peace; they're expected to keep busy: to perform miracles, to intercede. Being in jail or dead is no excuse for non-performance of the supernatural. One legend says, while awaiting his execution, Valentinus restored the sight of his jailer's blind daughter. Another legend says, on the eve of his death, he penned a farewell note to the jailer's daughter, signing it, "From your Valentine."