Abigail Pettigrew

Abigail Pettigrew was one of the people taken by Elliot Sardick as "security".

Frozen
Abigail was taken by Elliot Sardick and cryogenically frozen. This allowed him to barter with those who owed him money, in this case, her family. She was suffering from a fatal disease, the freezing prevented her death but when she awoke the process continued.

Christmas adventures
Abigail was awoken by Kazran Sardick and the Doctor. She was able to calm a sky shark that was hunting them by singing. She went with them when they returned the shark to the cloud bank. Abigail then returned to her case. Just before she froze Kazran promised they would visit each year.

Kazran and the Doctor woke her every Christmas eve and took her to various locations. Over the years as he grew older, she and Kazran fell in love. When it came to pass that she had one day left to live unfrozen, she told Kazran of her illness. She returned to her case. Kazran told the Doctor not to return next year as he was not going to wake her.

Final day
Many years passed and she remained frozen. After a spacecraft became damaged in the cloud bank on Christmas day, it was about to crash. The Doctor needed Abigail to sing, breaking up the cloud and allowing a landing. Kazran was reluctant; unable to choose what say should be their last together. She awoke and told him "Christmas day".

She sang into the Doctor's sonic screwdriver. This was transmitted to its other half in the shark. The cloud opened and the ship survived. Kazran and Abigail then spent her last day together, using the shark to fly across the sky in a carriage like they did in one of their first adventures with the Doctor. (DW: A Christmas Carol)