Walter was born, presumably in the decade of 1910's, into the ClanDestine-- his father, the immortal Adam Destine and his mother a magical genie. His history remains largely unrevealed, although he apparently enjoyed his childhood in the Clan.
Although he knew of his powers, he first used them publically after enlisting in World War II. There, his army was caught off guard by a huge mechanical robot designed by the Axis. The robot managed to bring down many of the costumed World War II heroes, the Invaders, right before Walter's eyes. He "hulked" out-- pushing his strength to levels never before reached-- to defeat the robot, only to have the reinforcements to his army attack him, thinking he was the threat. Walter never wanted to be a superhero after this experience.
Thirty years later, Walter agreed to the "Relative Strangers Protocol," as Clan members split across the globe. Walter apparently stayed in England, and at some point developed his career of writing romance novels under the "Sabrina Bentley" pseudonym.
Later, Rory and Pandora's crime fighting exploits caused a chain of events where the existence of the Clan was discovered. Walter then helped the twins learn of their legacy, and, with the help of other Clan members, defeat their enemies. This was when the twins gave their "Uncle Wally" the nickname Wallop.
Walter has since rejoined the rest of the family at the old family manor at Ravenscroft, and continues as guardian to the twins, often (and reluctantly) as chaperone to their crime fighting.