She was an old school friend of Grant's, but before Grant could confess his love, his best friend beat him to it and Lucy married him. However, once Mr. Lombard found out she was pregnant, he left. Grant became nanny to Lucy's daughter Jennifer, unaware that he still loved her.
When the Twelfth Doctor and Nardole arrived in New York investigating Harmony Shoal, the adventure led Lucy to finding out the Ghost was Grant and how deeply he cared for her. They decided to become a couple. (TV: The Return of Doctor Mysterio)
Lucy and Grant later married. Eight years after the Harmony Shoal incident, the Doctor returned to Lucy's life asking to take Grant on a quest through time and space looking for the three siblings of the crystal which gave Grant his powers. Grant refused to go unless he could bring Lucy and Jennifer with him.