For the longest time, Zack was known as Isaac Theo Carver, the long-awaited and much-beloved son of Abe and Lexie Carver. For years the two had tried unsuccessfully to have a child of their own with no success, until Lexie's father, Stefano, got them in touch with Marlo...a young pregnant woman. Abe and Lexie adopted the baby that Marlo gave birth to...or so they thought. They didn't realize that Stefano had arranged to have Marlo's baby switched with Hope's baby when it was born.

Both Abe and Lexie were thrilled with their new son...he was everything that they had ever wanted. Even when the baby Hope had was found to have fetal alcohol syndrome and Isaac was normal, no one really thought anything of it. Lexie fell deeper and deeper in love with her newborn baby, and when she learned that Stefano had switched the babies at birth, it devastated her. Hope was her best friend, but Isaac was the child of her heart. In her desperation to keep the child she craved, there was no limit to what Lexie would do.

Everything came to a head when the boys were almost two years old. Paternity tests proved that J.T., the baby Hope and Bo were raising, was actually the son of Glen Reiber and Marlo, which meant that someone else was Hope's baby. Originally Bo and Hope wanted to keep both babies, but when they lost custody of J.T. to his biological father, they fought all that much harder for Isaac. Abe was prepared to give the Bradys their son, but Lexie fought against it. Isaac had been with them for nearly two years and thought of them as his parents; why disrupt his world now? Unfortunately, when Lexie attempted to flee with her son, Abe arranged for a judge to give custody to Bo and Hope, and Hope was finally reunited with her son (who at the time they thought was not Bo's).

When Bo and Hope reclaimed their child, they renamed him Beauregard Isaac Theo Brady. Rather than put the child through any more trauma (switching parents mid-stream was bad enough), they opted to refer to him by the nickname Zack. They held a christening for Zack with all their family and friends present, which was almost ruined when Lexie burst in, but what no one realized is that Lexie knew the baptismal water was poisoned, and was trying to save Zack's life. Lexie was dragged off to the hospital, and the christening went on without a hitch.

Since the christening, Zack has been living with his parents and brother (until Shawn moved out), and has adjusted well to his new life. He was briefly left in the care of his brother, Shawn, when his parents were presumed dead, but now they're home, alive and well, and all is right with Zack's world. Zack was thrilled to discover he had a half-sister named Chelsea, and he accepted her unconditionally. Unfortunately, tragedy struck when, on New Year's Eve, Chelsea was driving recklessly in Bo's car and accidentally hit Zack, who had gone out to check on a cat. Zack never recovered from his injuries and Bo and Hope chose to take him off life support later that night so that he could give his liver to his niece Claire.

Bo and Hope continued to remember Zach by hanging his ornaments and his stocking up at Christmas. In the winter of 2008, Zach appeared in a vision to Bo to warn Bo that Theo might be in trouble.