Name: Sean Cassidy
First Appearance: X-Factor (vol. 3) #39 (Mar. 2009): "Multiple Birth"
Super? (Has Super Powers/Special Abilities/Technology): Yes
Number of Appearances: 1
Note: infant "son" of Madrox and Siryn; actually a Madrox "dupe"
Sean Cassidy appeared to be the infant son of Jamie Madrox (the Multiple Man) and Theresa Cassidy (the mutant super-hero known as "Siryn"). Theresa named the infant child after her late grandfather, the mutant superhero known as "Banshee."
Within hours after the baby was born, his father Jamie Madrox held him. As soon as he held his infant son, Jamie unintentionally absorbed the baby back into himself. Jamie Madrox then realized that he had not actually fathered a child, but that one of his duplicates has been the one who had slept with Siryn, and that the "child" of a duplicate is simply another duplicate. The "child" Sean Cassidy thus ceased to exist. The child's memories (what few there were) and lifeforce had been re-absorbed into Multiple Man just like all his adult duplicates are.
No amount of explanation could satisfy Theresa Cassidy, however. She blamed Jamie Madrox for killing her child and she threatened to kill him if she ever saw him again. Jamie Madrox left Theresa and the other members of X-Factor and began a long period of solitary despondency.