Something that's rarely talked about is the duality of Vanitas, who's constantly related to the cold (his cold emotions, Lunas words to him, as well that he seems to be freezing quickly) and Ruthven, who's tied to warmth due to his fire abilities. It's a playground for so many things. Vanitas hating to show vulnerability, but if he's freezing, he can't hide it.
If he's in Altus, coincidently during an especially cold night, and Ruthven notices that the fragile human, who just pretended to know no fear and is pretending to be so strong and invulnerable, is shaking next to him, trying to find comfort in the thin bedsheets. At the beginning, his sole reaction must've been to just get a better blanket, surely the human will be content with that. But as it goes on, Ruthven might consider to push the boundaries further, pull Vanitas closer, and use the warmth of his body to help the human through the night.