William was conceived by Tigra and Criti Noll, a Skrull agent posing as Hank Pym. Tigra gave birth to her son within two months and left him with the Cat People who gave her her powers to protect him from Norman Osborn, who desired to experiment on him. Later analysis determined that William is fully human, with a bit of cat DNA, given that Criti used Pym's DNA while undercover.
William appeared with Tigra at a birthday party for Nadia Van Dyne. After Tigra admonished Viv Vision for nearly revealing his Skrull heritage, Nadia happily accepted William as her half-brother, which Tigra tearfully accepted.
- William's existence was first mentioned in Avengers: The Initiative #20 where it was revealed by Tigra. Trauma also shaped as Tigra "giving birth" to a litter of Skrulls coming through her corpse, a representation of Tigra's fear towards her unborn baby.
- As Nadia accepted William as her half-brother, it is highly possible that he is now officially connected to the Ultron side of the family, including Vision, Vivian, and Jocasta.
- Given the sliding time scale and William's accelerated maturation speed from his Cat DNA, William is approximately two years of age with the physical maturity of about five years growth.
- William is most likely named after Tigra's deceased husband, William "Bill" Nelson.
- Similar to his mother's debut as Tigra, William's debut was in the Chinese Zodiac Year of the Tiger.
- 7 appearance(s) of William Grant Nelson (Earth-616)
- 1 minor appearance(s) of William Grant Nelson (Earth-616)
- 1 mention(s) of William Grant Nelson (Earth-616)
- 2 image(s) of William Grant Nelson (Earth-616)