Traditionally we don't touch upon apocryphal portions of the Castlevania continuity on this site. Things that are referenced in random notes from designers, or come from one instruction booklet or another, are generally ignored because, if it can't be backed up by events in the game then we can't trust it's an actual part of the continuity. We have to see it, in effect, to know it's true.
However, there are some details that come pre-baked into the characters and the story where, even if they never appear in the games, we know it had to have happened. We know that the Belmonts train to be vampire hunters, for example. We don't need to see it, or be told how often they practice, to know this detail is true. They kill vampires with whips, they back up their claims with hard evidence in the form of even harder ass-kicking. That's all the proof we need.
In the case of the back story for Eric Lecarde, we know some details are true. His was motivated to chase after Elizabeth Bartley (ne Erzsabet Bathory) after that vampiress killed his fiance. That's the motivation to get him to Castle Dracula and set him on his own quest to defeat the Dark Lord once and for all (or, really, for a time until the next generation comes along, as so often happens in this series). The only detail missing was: who was the girl.
Obviously there had to be a girl. We know this. In the game she goes unnamed. In a fax to Electronic Gaming Monthly, the name of the fiance was given as "Lucy Seward" (an obvious combination of two characters from Bram Stoker's Dracula, Lucy Westenra, who is killed by Dracula in the novel, and John Seward, one of the men who takes on the vampire and avenge the dead Lucy). In the U.S. instruction booklet, she's named "Gwendolyn". Obviously there's some confusion there.
Again, we know she existed inside the continuity of the series. We at the Inverted Dungeon, then, choose to give her the full name of Lucy "Gwendolyn" Seward, acknowledging both sources without choosing one over the other as "official". She's here, as part of the continuity, even if it was but a small part.
Lucy Seward ("Gwendolyn" to her friends for some reason) was betrothed to Eric Lecarde. He was a distant relative of the noble Belmont clan, and she was related to one of the men who had taken down Dracula years prior in England. From a certain perspective it was a perfect match, aligning two vampire hunting vampires in matrimony. Unfortunately for their long-planned bliss, a vampire got in the way.
While Eric was off on a journey, the evil Elizabeth Bartley arrived at the Seward family home to have her own revenge. The Sewards had killed her uncle, so she would eliminated their future. She attacked young Lucy, draining the woman of her blood, killing her. She left the body, knowing the fear and anguish it would cause, relishing that pain. What Elizabeth put in motion, though, was a tale for vengeance.
Once Eric heard the news, saw that his betrothed was dead, he swore he would kill Elizabeth. He went to his family's ancestral estate and picked up the powerful spear that was designed to hunt vampires. He then went to Castle Dracula to give Lucy the peace in the afterlife she deserved.