The better question is ‘who were?’ Depending on who’s counting, there are between 12 and 14 Saint Valentines, including a Spanish hermit and a woman – Valentina.
As far as anyone can tell, the Saint Valentine of Valentine’s Day was one of two guys preaching the good word in Rome in the third century. One of these two was martyred on February 14th 269, thus giving us the date for his eponymous day. After he was killed, Valentine’s remains sat in the Catacombs of San Valentino for a while before moving to Santa Maria in Cosmedin where they were visited by pilgrims for many years.
So Valentine’s Day is basically a sham invented by a poet in order to make his lines rhyme? Not exactly.
Shortly after Chaucer mentioned love on Valentine’s Day, real life lovers began to send each other love poems on February 14th. To put that another way, people have been writing valentines to their loved ones for over 500 years, so even if there’s no direct connection to a guy who was murdered horribly by ancient Roman pagans, it’s still part of a serious legacy of love.