Happy Saint Valentine's Day!
What do we know about Saint Valentine? Almost nothing. Here is a History.com article which is a pretty good summary and agrees with all the evidence I have from other sources.
The Catholic Encyclopedia, which is usually thorough to the point of verbositiy, doesn't find much to say about Saint Valentine. Indeed, there were at least three saints by that name and we know next to nothing about any of them. But if you want a summary of the pious version here it is at Catholic.org.
The legend that one Saint Valentine fell in love with his jailer's daughter has been embellished in popular culture. We don't know if the legend is true and if it is true whether the love was romantic and if it was romantic whether it was consummated. We have virtually no evidence one way or the other. The best evidence is that there was no torrid love affair because the very legends which tell of his "love" for the girl are perpetuated in an environment which would have denied his sainthood if he had an affair. It's much more exciting though, to trash a Saint, so whatever. The whole modern myth stems from an older legend that he miraculously restored her sight and then, on the eve of his execution, sent her a goodbye letter signed "from your Valentine". However, this embelishment was added in the 13th Century, some 800 years after the establishment of the feast in his honor. The romantic associations come from the legend that he was imprisoned in the first place for illegally conducting Christian marriages and otherwise aiding persecuted Christians under Claudius II.
The Wikipedia articles on Saint Valentine and his feast day seem pretty accurate. A quick perusal did not reveal any typical nonsense (it's usually easy to spot anyway).
Though the association with romantic love is probably most due to Chaucer's work up of an embellishment of a legend, I'm happy to join the fun and send my girl a Valentine card telling her how much I love and appreciate her. I doubt Saint Valentine will mind.