Spencer Stefan Nikolossovich Cassadine (born John Michael Jacks) is born in February 2006 to Nikolas Cassadine and Courtney Matthews. He was originally believed to be the son of Jasper Jacks. On Courtney's death bed, Jax revealed to Courtney that Nikolas is the father. Jax chooses to keep the secret and raise the baby as his own son. Naming the baby John Jacks in honor his late father, Jax soon discovers that Carly Corinthos learned the truth. Carly aggress to keep Jax's secret and offers to help raise the baby with him. Robin Scorpio also learns the truth about the baby's paternity, and during the baby's christening ceremony, reveals the truth to Nikolas. Returned to his real father, John's name was changed to Spencer Cassadine in an effort to show the conclusion of the Spencer/Cassadine feud as well as addmit his ties to that family via Lucky and Lulu and their father Luke.
The child's great-grandmother, Helena hires Collen to become the baby's nanny. Colleen develops a crush on Nikolas and becomes desperate for the two of them and Spencer to become a family. Under disguise, Collen kidnaps Spencer from the manger on Christmas Eve and changes the baby's name. Nikolas and Emily Quartermaine travel to Moscow to find Helena christening him with a Cassadine name. Helena then holds a knife to Spencer. Emily punches her and takes the baby. Helena escapes while they are waiting for the police. Spencer returns to town with his father and Emily where there is a party thrown for him. When Nikolas begins to hang out with Nadine Crowell, Spencer shows her his hores and then helps his father and Nadine put together his train set.
Spencer is rarely seen, and becomes clear that Nikolas is leaving him to be raised mostly by his nanny. People have admitted that Nikolas appears to be an absentee parent. Since then, Nikolas has been more active in his son's life. Spencer has been seen in Wyndermere and at the park. Spencer later becomes a big brother to Aiden Cassadine.
- Caden and Nicholas Laughlin (2006)
- Nathan and Spencer Casamassima (2006-2007)
- Lance Doven (2008)
- Rami Yousef (2009)
- Davin Ransom (August 13, 2009-present)