'Just stay together,' he murmurs quietly, from the back.

Because X is fast, but if they get seperated and have to be tracked down, there's no guarantee of what might happen while they're waiting to be found.

He tries to keep Amy in sight. A little difficult from the back, and he's looking around all the time to check there aren't new threats arriving from within the trees. A few trunks get small black devices stuck to the side. If they do happen to get lost, a couple of explosions might lead them back on track. Or deter anyone following.