May "Mayday" Parker (aka Spider-Girl) is the only child of Peter and Mary Jane Parker in a future, alternative universe. Peter and Mary Jane were reunited with their baby daughter by Kaine, who found the child living with Alison Mongrain, the con artist who had kidnapped the baby on instruction from the Green Goblin. After they were reunited, Peter lost a leg during the horrific final conflict with the Green Goblin. After the battle, Peter was offered a bionic replacement by Mr. Fantastic and, considering it a wake-up call, decided to retire and focus on being a husband and father. For years, they chose to keep their past from Mayday and hoped that she wouldn't develop powers of her own like her father.
Healing from an alternate future, Spider-Girl has traveled to the FusionFall universe to help save the Earth from Fuse, because her world was about to be attacked by a future version of Fuse, who was set to attack many other alternate universes. She's on the Helicarrier to convince those there to destroy Fuse while they still can, but Nick Fury is dissuaded by Fusion Firestar, who he thinks is the real one.
Spider-Girl gives the player three missions: her Nano Mission and two World Missions, one to defeat Fusion Firestar in her Secret Lair and one to get the Webshooters.