He lost her after the first surge of sentinels, but early on in the days of the mutant purge. He got the kid out of the mansion when the robots first targeted it, back in the days when everybody just barely understood that the government was trying to completely eradicate them. He felt her arms tighten around him as the rode away on his motorcycle, away from the genocide that was happening just miles away. He could smell the blood.
They went to his cabin in Canada. Neither of them realized the capabilities of the sentinels. They were found within the month, but he got them out of there. The sentinels had yet to evolve, as he would find they had later: there were no laser beams, or other advanced attacks, stolen from mutants they encountered. They were simple, straightforward killers.
The girl was scared, but she hid it behind her white streak and strong persona. She didn't know who was alive or dead, and she dreamt every night of a familiar face buried under the rubble of a mansion she called home. Her savior gave her courage to continue on, even as they were on the run.
They managed 6 months together, packing up every two or so weeks before the robots could trace them to whatever obscure location they lurked in. They had no idea that the sentinels could evolve, and they had no idea that those monsters were closing in on them. They thought they were safe in the roadside motel, in the middle of nowhere. They had stayed a week and felt confident for a few more days, at the very least.
The man was coming back from picking up supplies when he heard the screams. Her voice screeched for him. Her poison skin may have been a useful defense against humans, but metal could touch her. It could rip her. By the time he reached her, the mutant hunter had moved on, believing it had eradicated its target. As he cradled her in his arms, just as he had years ago on that island, he wept. There was no poisonous pull from her, no matter how many hours he pressed his face against hers.
"I promised to protect ya kid." He whispered against her blood-soaked hair. When he left that motel, her blood still covered him and her screams echoed in his ears.