The Red Hulk tells Bruce Banner how Thundra came back in time to fight the Hulk. She did so to inspire her forces, but also to get a few cells from the Hulk to conceive a powerful baby. She got such cells by kissing the Hulk (in Hulk: Raging Thunder #1). Scientists used those cells to get Thundra pregnant, conceiving Lyra, the Savage She-Hulk, offspring of Thundra and the Hulk.
The Red Hulk also tells Banner that Lyra is currently in our timeline. And that the Wizard enlisted her for his Frightful Four team.
What the Red Hulk doesn’t tell Banner is that Thundra could not return to her time after her battle with the Hulk because her time machine failed. The Intelligencia wanted to use Thundra in their schemes. Knowing his bond with the Intelligencia was short-lived, the Red Hulk looked to Thundra as an ally after their fight (with the Lady Liberators group Thundra was a member of, in Hulk #9). The Red Hulk promised Thundra to help her return to her time in returned which happened when Doc Samson and the Red She-Hulk attacked the Red Hulk (in Hulk #17).
Wizard, Claw and the Trapster tried recruiting Thundra for capturing Reed Richards, but she energetically declined their offer (she’s a friend of the Fantastic Four). When the villains fought her, the Red Hulk appeared, trashed them, and rescued her.
After the Cosmic Hulk kidnapped Dr. Doom for the Intelligencia (in Incredible Hulk #606), the Red Hulk brought Thundra to his castle to use Doom’s time machine to return Thundra to her time. Vampire keepers almost defeated them if not for Thundra, who used the machine to bring them into a sun lighted desert. Finally, Thundra could return to her time (and conceive Lyra) but not before thanking the Red Hulk for his help with a “smack” (kiss in the mouth). The Red Hulk wondered if she got cell samples of his as well... A-Bomb, undetected, was spying on him.
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