The Red Hulk tells Bruce Banner how Thundra came back in time to fight the Hulk to inspire her future forces, but also to get cells from the Hulk and conceive a powerful baby; she gets the cells with a kiss (in Hulk: Raging Thunder #1). Then, scientists from Thundra's time get her pregnant and thus Lyra the Savage She-Hulk, daughter of Thundra and Hulk, is born. The Red Hulk also tells Banner that Lyra is currently in our time, and was enlisted by the Wizard for his Frightful Four team, and needs to deal that.
What Red Hulk doesn't tell Banner is that Thundra was unable to return to her time after her battle with the Hulk because of problems with her time machine. The
Intelligencia wanted to use her for their plan. Feeling his bond with the Intelligencia was not to last, the Red Hulk looked for Thundra as an ally after their battle (with
Lady Liberators group Thundra was a member of, in
Hulk #9). The Red Hulk promised to help Thundra get back to her time if she helped him... Which happened when
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 of the Fantastic Four, but she energically declined their offer (she's a friend of the super hero team). When the villains had almost defeated her, Red Hulk came and trashed them, rescuing 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. Vampires keepers almost defeated them if not for Thundra who used the machine to bring them into a sun lighted desert. Finally, Thundra was able to return to her time (to conceive her child, 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
A-Bomb, undetected, was spying on him.