Born Amelia Jane Coffey in early 1870, Tennyson was renamed by her sire, Caius, after he turned her. He had been reading a newly published poem titled Now Sleeps the Crimson Petal by Alfred, Lord Tennyson and found that the verses reminded him of his new fledgling bride.
Caius was turned by Angelus in Rome during his stay with Darla there in 1771. He had originally been employed by Daniel Holtz to assist with tracking down the two vampires who killed his family, but Angelus and Darla tracked Caius down first, turned him, and sent him back to Holtz as part of their many attempts to ruin his life. Holtz had already returned to England, though.
A century after he was turned and had been busy cutting his own dark swath through most of the world, Caius found Amelia's family shortly after her birth. He planned on killing them for the anniversary of his own siring but was enchanted by Amelia's pale skin, dark eyes, and dark hair. He thought of turning the baby and bringing her home to his current girlfriend, Rosemary, who had been lamenting her lack of child or childbearing abilities. Instead, he struck a bargain with Amelia's parents that he would let them live as long as he could come back for Amelia on her 21st birthday and they wouldn't resist him. Several months later, the family moved and went into hiding in an attempt to lose the vampire's track. But Caius found them and on her 21st birthday, he killed Amelia's parents for trying to betray their deal and turned her for his own.
They spent the next century together until Caius was presumably killed by Angel in 2001. In the past 7 years, Tennyson has refused to consider any other vampire as hers and is bitterly seeking and planning revenge on Angel by removing his soul and making Angelus her new vampiric husband as punishment to Angel's soul and because it is the second best option since she can't bring Caius back.