Write about losing someone or something you were close to.
It all happened before Kei had time to think. One moment he and Alyx were perched on a rooftop looking for a route across the canal, and then suddenly Alyx was surrounded by Overwatch soldiers and shouting for Gordon to get away. Of course, Kei rushed to defend her; whatever the Overwatch soldiers might be, they're no longer men, and the tiger daemon has no qualms about tearing into them with tooth and claw. The soldiers didn't react to his approach--they never do, they don't even seem aware that he's there--but the first one he struck flailed wildly enough to throw him off its back, and off the roof of the building.
It wasn't a long fall to the next rooftop--two stories at most--but it was enough to knock him unconscious for a few minutes. Long enough for the Overwatch to have Alyx loaded into the dropship that's taking off now.
Even before he fully regains consciousness, Kei is overwhelmed by the feeling of separation. He's never been this far from Alyx before, and he's getting further away by the second as the ship rises into the sky. He frantically sprints and leaps from rooftop to rooftop, his eyes never leaving the dropship. But even at his fastest he can't match the speed of a Synth, and it's traveling upward as well as forward, and oh God it hurts it hurts it's like being turned inside out Alyx I'm sorry come back Alyx PLEASE--
His roar of pain and anguish echoes for miles around. And still, Kei keeps moving forward in the path of the ship. He can't not.