Was Ethan a hundred percent certain that he wanted to show Lillian memories of his past life?
No.
He wasn't sure what the outcome was going to be, was she going to realize that she was only by his side because she was useful?
Still, he'd prefer if she knew now, especially given the fact that they were about to face a technologically advanced alien race.
If he had been able to create a memory extraction helmet, he wouldn't be surprised if they had the same tech but a lot more advanced.
So he'd rather tell her himself, than have his memories extracted and shown to her, though the chances of that happening were extremely low, they weren't zero.
"Alright then," he muttered as he summoned the simulation's control panel, "let's show you my memories, shall we?"
Lillian was a bit surprised, especially given the fact that she expected them to exit the Simulation and prepare for the incoming alien.
