Lisa

Although Lisa has never physically appeared in any of the games, she was referred to in at least one game, Castlevania: Symphony of the Night.

During the years before 1450, the Count Dracula met the human woman Lisa and started up a relationship with her. Due to the union between the two, Lisa would give birth to a son, Adrian Fahrenheit Țepeș, who would be better known later in life as Alucard. Adrian, who loved his mother very much, attended her burning death and heard her last words.

Lisa was a healer, a medicine woman, but she would come to the end of her life when she was accused of being a witch and her expertise in medicine is mistaken as "witchcraft". At her side was Adrian, to whom she told her final words, "Do not hate humans. If you cannot live with them, then at least do them no harm. For theirs is already a hard lot." She also told him to tell his father that she "would love him for all eternity." Upon learning about his wife's death, Dracula vowed revenge against humans as Alucard did not get the chance to tell his father Lisa's last words until the year 1797 when they met in battle.

It should be noted that Lisa is only ever referred to by her first name in the games; neither her maiden name nor what name she took when married (Cronqvist, Țepeș, or a pseudonym of some kind) have been established.

Symphony of the Night
In Castlevania: Symphony of the Night, the Succubus disguises herself as Lisa in Alucard's nightmare, in order to get Alucard on the side of evil. However, her disguise was spoiled when she told Alucard to kill the humans that wanted to execute her. Alucard, knowing his mother would never say such a thing, saw through the demon's trick. Enraged, he fought the monster and promptly slew her, saying "Death in the dream world will send your soul wandering for eternity, demon." It seems odd that Alucard wasn't aware of the deception earlier, when Lisa (Succubus) called him "Alucard", as his name is "Adrian" (But he may not have noticed after following the name "Alucard" for hundreds of years).

Speculation

 * It seems odd that, after roaming "foreign lands" for approximately 400 years whilst cursing the name of God, Mathias/Dracula would take a human for a lover. It is to be noted that there is some resemblance between Lisa (Adrian's mother and Dracula's lover) and Elisabetha, Mathias's wife from Lament of Innocence. This would suggest that Lisa is a possible reincarnation of Elisabetha. This would make sense as to have Dracula stop his cursing of God and roaming of the earth in anger, to falling deeply into the thralls of love he once knew; he was even able to bear a child with her. It would also make sense that once she was wrongfully killed as a witch and once again taken away, it would spark again the anger in him to destroy God and mankind. This may have been the catalyst leading to the war on mankind that Trevor Belmont faces in Castlvania III: Dracula's Curse.
 * It is quite interesting as to how Lisa resembles Leonardo da Vinci's "Mona Lisa" another one is when the reader looks closely at one page in the SoTN artbook manga, they might see a portrait of Lisa sitting in the same position as Leonardo da Vinci's work.