It happened the very first time they Drifted. Raleigh had dragged her down, out of sync and down a RABIT trail that she couldn't help but follow. Once she'd caught the tail of the monstrously strong RABIT that was her worst memory, there was no letting go and she sank down, down into the past.
Raleigh had shouted at her, aloud and in her mind. "Stay focused, stay with me, Mako!" But his voice was lost in the thundering of her heartbeat in her ears and the echoes of Onibaba's footsteps in her mind. She couldn't hear him, she couldn't listen, everything was too loud...
"Mako. Mako, you're doing great. Stay calm and try to focus. None of it is real. Mako, do you hear me? Calm."
The voice wasn't Raleigh's and it wasn't Pentecost's and it wasn't any voice she'd ever heard. But she knew that voice. She held onto that voice like a lifeline, drawing herself back, hand over hand. She was going to make it. It was going to be over.
Then Onibaba had turned and caught sight of her. Mako lost the line, fell back into the memory, and ran screaming until it ended at Coyote Tango's feet.
The whole Drift was a catastrophe and the aftermath was even worse. It wasn't until that night, alone in her bunk, that she remembered the voice. She knew the voice, and in the dark, she gave it a name.
"Yancy."
In the Drift, two minds are fused. The pilots are literally inside each others' heads, the better to operate in tandem and smash the kaiju. Normally this works out well. Occasionally there are side effects, withdrawal or ghost drifts. And sometimes, when one pilot is violently killed while still Drifting, a piece of their consciousness is preserved in their partner.
It happened again every Drift after that. On the surface, she could hear Raleigh, see him and feel him and work with him. But underneath there was Yancy. It was very surreal, speaking to a dead man in the Drift. She wondered if every pilot felt the Drift on two levels, a higher functioning level and a background layer.
Yancy was her background layer, the calm counterpoint to Raleigh's excited puppy mind. Where Raleigh got hyped up and shouted during the battle, Yancy was ultimately a lot more helpful, whispering encouragement and keeping her on track. She grew used to his voice and his help, focusing on his quiet words as she helped Raleigh bash in Leatherback's brains.
She wondered too if Raleigh could feel his brother or if he even knew he was there. She guessed not. She could feel the hole in his heart where Yancy had been, and she could feel Yancy's lingering spirit's longing for his brother. They'd never gotten to say goodbye. Yancy had been dead for years, only his echo remained. He was not truly there, only his ghost remained in his brother's mind and in the Drift.
But the remnant was strong and his voice was ever in her mind, in the Drift. Raleigh was her adrenaline and Yancy was her anchor and when they Drifted, she felt whole.