It is possible that you might be thinking of:
* Saint Alexandra of Rome: She is a figure mentioned in some Christian traditions, but her existence and story are considered largely legendary. There's no definitive historical evidence to support her existence.
* Saint Alexandra of Constantinople: There is some information on a saint with this name, but she is not widely celebrated. Information about her is limited and her historical authenticity is uncertain.
* Another saint: You might have encountered a name that is a variant or mistranslation of a different saint's name.
Could you please provide more information about where you encountered the name "St. Alessandra?" This would help me understand what you're looking for and perhaps identify the correct saint.