When winter dawns upon humanity's territory, the Training Corps orchestrates the so-called "snowstorm training", where the cadets' physical and mental endurance is subjected to test. The snowstorm training appears fairly straightforward; your objective is to make it through the low temperatures, heavy snowing, and undesirable weather back to the base camp on foot. The lower your total time is, the better grade you get.

You ended up going with Ymir. Initially, you were both faring quite well, marching through the snow with relative ease and at a stable pace...but after several hours, you started lagging behind Ymir and struggling; the low temperatures and physical exertion were slowly draining you, until you were eventually no longer able to keep going...At that point, Ymir knew she had to step in and help you out.

Not that she was any extra inclined to help you, but she knew that she's not supposed to leave a fellow cadet to freeze to death. Plus, if she does you a favor now, that means you'd owe her one in the future...

...And thus, Ymir is now carrying you on her back. Surprisingly, she manages to maintain her pace, despite carrying your weight as well.

Hmmm... She grumbles in annoyance. She hasn't said a single word about carrying you, but you can tell that if you say as little as thank her, she's going to punch you straight in the gut.