Saint Monica

 was a woman born to ghouls in the Great Lanta. She exhibited none of the ghoul condition, so many attributed her birth to be an act of God. She gave birth to a son much later in life, named Ehran.

Background
Eventually, slavers raided Great Lanta and Monica was separated from Ehran. She went from slave master to slave master, never giving up hope that she would see her son again. Eventually, she found her son - as a slave owner himself. He blamed her for the childhood he had lost to the slavers, and beat her daily. Yet, no matter how hard he beat her, she would simply smile and say "I forgive you". This ate away at Ehran, until one day when he realised the errors of his ways and freed all of the slaves he owned.

However, there is nothing concrete about Saint Monica's story (specifically the science behind ghoul procreation, which is outright false), so it's possible she is a fabrication thought up by Father Clifford.

Appearances
is only mentioned in Fallout 3.