You can make these kinds of pairings based on a lot of different traits. It doesn't always have to be as literal as "they're both yuki-onnas." In this case it's because Letty only shows up when it's snowing, and it's always hailing when Abomasnow appears on the field. Personally, I think that's more fitting.
gustwind said: Letty's not a yuki-onna if that's what you're implying. She's the spirit of winter.
