According to The Golden Legend (Aurea Legenda), which records many fanciful stories of the lives of the saints, Saint Valentine was a martyr, executed by Emperor Claudius for denying the Roman gods.
According to the Catholic Encyclopedia, there were at least three such Saint Valentines, all martyrs, all “mentioned in the early martyrologies under date of 14 February.”
None of them had any particular relevance to lovers — except for that date:
The popular customs associated with Saint Valentine’s Day undoubtedly had their origin in a conventional belief generally received in England and France during the Middle Ages, that on 14 February, i.e. half way through the second month of the year, the birds began to pair. [...] For this reason the day was looked upon as specially consecrated to lovers and as a proper occasion for writing love letters and sending lovers’ tokens. Both the French and English literatures of the fourteenth and fifteenth centuries contain allusions to the practice.