BIOGRAPHY - PAGE 1
The genesis of Billy Kaplan begins with the death of Mephisto, except that didn't happen and none of this is true.
The Lord of Lies grew arrogant at the onset of the Wraith War, the arrival of the malignant sorcerous beings known as the Dire Wraiths who invaded Earth in the presence of their own horror-filled satellite, Wraithworld. Mephisto's ability to feed on the ambient evil of Earth increased his power dramatically and he brought it to fore in a battle with Mr. Fantastic and the Invisible Woman of the Fantastic Four. As he brought their souls to his realm for torture, however, Mephisto took care to imprison their son Franklin Richards. Franklin's vast psionic powers were inhibited by his waking mind in his physical body, but those safeguards would do nothing to restrain his spirit form. Reed Richards recognized this and bade his ally Doctor Strange to free Franklin. Franklin's unreserved power was turned on Mephisto just as the Wraith War ended, robbing Mephisto of his extra power and leaving him dangerously vulnerable. The Scion of Hell was apparently completely and utterly destroyed by Franklin's powers. [Fantastic Four (1st series) #276-277]
Richards and Strange foresaw Mephisto would return so long as evil continued to exist on Earth, however. They were correct, and Mephisto did resurface in short order, although he was now greatly diminished from his previous stature. Five fragments of Mephisto's power were scattered throughout the Earthly planes of existence, requiring him to recover them in order to regain his old might. Instead of pursuing the shards directly, Mephisto conceived a fiendish plot to trick a human agent into recovering them for him. A man named Martin Preston lost an arm and faced death in a car accident, and offered up his soul to Mephisto in exchange for salvation. Mephisto restored Preston and gave him powers, but lied that he had torn Preston's soul into five pieces and challenged him to reclaim the shards and his humanity. Thus, Master Pandemonium was born as Mephisto's unwitting agent in recovering his lost power. [West Coast Avengers (2nd series) #4,9,15, Avengers West Coast #52]
Around the same time, the married couple of Wanda Maximoff and Vision of the Avengers became interested in starting a family. Their desire to become pregnant was stymied by Vision's nature as a synthezoid, lacking the necessary materials to make a baby. During a conflict with Salem's Seven, Wanda had the opportunity to channel the mystical power of an entire town of 
witches through her body, with Vision physically bracing her for the spell. Taking advantage of the situation, the Scarlet Witch augmented her probability-manipulating power with this mystical energy, seemingly causing the mind-bogglingly improbable event of her pregnancy to occur. Wanda was with child and, nine months later, Doctor Strange delivered her and Vision's twins. Thomas and William were named after Phineas Thomas Horton (creator of Vision's body) and Simon Williams (template for Vision's mind.) [Vision & the Scarlet Witch (2nd series) #3-12]
This joyous occasion was not all it seemed, however. At its core, Wanda's pregnancy began as an example of pseudocyesis, a so-called "phantom pregnancy" when a woman is so desperate to 
conceive a child that her mind "tricks" her body into physically manifesting the secondary signs of pregnancy. A child typically does not and cannot be birthed in these situations, but Wanda complicated the situation even further by introducing witchcraft to the equation. Through the power of New Salem, she subconsciously reached out into the universe and snagged two shards of Mephisto's living power to act as anchors for her children. Therefore, it was these mystical energy fragments Wanda gave birth to, and not real flesh-and-blood children. [Vision (3rd series) #7, Avengers West Coast #52]
The difference between "real children" and what Wanda had might have been academic (or not) given time. Thomas and William were lovingly raised as infants by their parents at their home in Leonia, New Jersey. The twins enjoyed visits by their Uncle Simon and their grandmother, Simon's mother Martha. They later moved to Avengers Compound in Palos Verdes when Vision and the Scarlet Witch rejoined the Avengers on their west coast roster. [West Coast Avengers (2nd series) #30, 40, Annual #1]
Two aspects of the twins' existence proved telling for their fate, however. First, the boys only existed for as long as Wanda was consciously thinking about them. Whenever she was distracted 
or focused on other things, Thomas and William winked out of existence. A series of governesses had considerable trouble explaining to Wanda how they kept "losing" the boys, only for Wanda to immediately find them when she came home. Even Agatha Harkness, returned from the dead, was unable to convince Wanda of her sons' unique nature. [West Coast Avengers (2nd series) #42-46, Avengers West Coast #47-50]
The second difficulty came from Master Pandemonium's continuing quest for his own missing soul. The demon sorcerer kidnapped the infants and merged with them, delighted to have apparently discovered two more fragments of his missing soul, and tremendously greater power as a result. Wanda was horrified to watch her two children fused into Pandemonium's body. 
Agatha Harkness arranged for the Human Torch to deliver the final soul fragments to Master Pandemonium, only for all his power to be extracted by Mephisto, who remerged with his lost quintessence. Harkness then took advantage of Wanda's connection to her sons' soul shards by causing her to forget the existence of her children, ripping away 2/5ths of Mephisto's power and forcing him into retreat. [Avengers West Coast #51-52]
This then appeared to be the final word on Thomas and William. Although the Scarlet Witch eventually was allowed to remember her children, [Avengers West Coast #71, Annual #7] they never returned to her. The boys seemingly no longer existed except as part of Mephisto's overall essence. Wanda recovered from their loss in the years that followed, but backslid when she began channeling the nigh-omnipotent power of Earth's Life Force. Her probability-altering powers were augmented to the point of being reality-altering power. Portions of her mind began lashing out at her fellow Avengers, and restored versions of her lost children. This power drove Wanda over the brink of madness and, after creating then dispersing the alternate reality bubble known as the House of M, Wanda vanished into obscurity under a cloud of amnesia that prevented her from remembering the vast power at her fingertips. [Avengers (1st series) #500-503, House of M crossover, Avengers: The Children's Crusade #6-7]
Enter Billy Kaplan. Billy was the Jewish son of Jeff and Rebecca, had two younger brothers and lived on the Upper West Side. After being roughed up by a bully at his high school, Billy ran into Scarlet Witch in Central Park the day before her chaos rampage. They talked briefly about being true to who you are and Wanda used her chaos magic on Billy, healing his injuries. The next day, 
Billy discovered he had the power to make things happen. Using repetitive incantations to focus his ability, Billy referred to his power as spell-casting. He was just beginning to understand it when he was approached by Nathaniel Richards. The teenaged incarnation of Kang the Conqueror from the 31st century, Nate was using Avengers Failsafe protocols found in the Vision's operating system to assemble a group of young Avengers to help him. For reasons neither of them understood, Billy was part of those protocols. [Young Avengers Special #1]
Billy, Nate and the other recruits decided to pattern themselves after their heroes in the Avengers. Nate altered his neuro-kinetic armor to become Iron Lad, Eli Bradley (grandson of the black Captain America from World War II) easily slid into the super-soldier motif of Patriot, and the super-strong shape-changer Teddy Altman posed as a gamma mutate named Hulkling. Billy chose the identity of the Asgardian, limiting him public spell-casting to flight and lightning spells to evoke a comparison to Thor. Although Iron Lad was certain Kang would be coming after him to reclaim his armor and reassert his original timeline, these "Young Avengers" spent their first few weeks training relatively peacefully for an enemy that didn't show. In the meantime, they met in the abandoned remains of Avengers Mansion to practice for the day Kang did show. Billy and Teddy hit it off as a couple and quickly started dating. [Avengers: The Children's Crusade - Young Avengers #1]