Xiang sat grumpily on Kat's shoulders like a kid who was told he couldn't get any candy at the store. He wasn't trying to sleep. He wasn't trying to be lazy. He was 'the lookout'. That's what he told himself anyway after being browbeat into letting Kat carry him while they ran. He didn't feel at all comfortable sleeping for the moment despite the creeping fatigue. They were walking through hostile terrain and the visibility was shit. Yes, the need for someone on watch wasn't a surprise at all.
Of course, Xuena could use the snowstorm itself to get a good sense of threats nearby and was using that talent to steer them away from the occasional spirit beast on their path. They didn't have the time or the strength to properly fight them in this sort of weather. It also wasn't entirely clear, at least in Xiang's mind, if Kat's fire would do anything to these beasts considering their ice immunity.
