Carl Holliday is the son of Jimmy King and Juliette Holliday. Carl was conceived after a mix up at the fertility clinic which meant that Juliette was given the wrong man's sperm. Carl lives with his father Jimmy, step-mother Nicola and his half-siblings Elliot and Angelica.
Juliette tracked down Jimmy so he could sign the paper that waved his rights to her unborn baby but she ended up going into labour at Jimmy's house, Mill Cottage. Jimmy's wife Nicola helped Juliette deliver a baby boy, who she named Carl, unaware that it was also the name of Jimmy's deceased brother. Juliette initially allowed Jimmy to have access to Carl but that stopped when she and her estranged husband Greg reconciled and decided to be a proper family. Nicola guilt-tripped Juliette into allowing Jimmy to see baby Carl for one last time, but Nicola took him and ran away. Nicola returned not long afterwards and Juliette was persuaded not to press charges against her.
In January 2016, Jimmy called Juliette to check in on Carl. Juliette appeared in the café and gave Carl to Jimmy explaining her husband had left her again. Jimmy agreed to look after Carl until Juliette had put her life back together. A few weeks later, Juliette agreed to give Jimmy parental responsibility as Carl was staying with Jimmy and Nicola on a more permanent basis.
After struggling to juggle childcare, Nicola became a stay-at-home mum to Carl, and his half-siblings Elliot, and Angelica, but she soon became fed up and she and Jimmy switched roles so he became a full-time stay-at-home dad.