I suppose it might have something to do with context and aesthetics, at least within Fruits Basket, there's only one of each, so you wouldn't need to pluralize. Conventions sure do make things interesting.
Incidentally, in Japanese, normally you wouldn't add anything--you'd just figure it out from context. But sometimes the authors have fun by adding a katakana zu, to add the English plural s sound, like with the Tomodachis in Fruits Basket and the Shinigamis in Full Moon o Sagashite. But, since it's only written in katakana, the question about -s or -es remains.
Changing the subject completely, in the library today, we had a bunch of people come in and ask for stuff, find out we didn't have what they were looking for (even though we're probably supposed to) and say, "That's okay; I'll just get it from this other place." So we're kind of like, "If it's all available somewhere else, what's even the point?" and were feeling kind of useless. Then a member of the bishopric came in, and when we asked if he needed anything, he said no, he just wanted to thank us for doing our calling, because it really is very important. We have many reasons to distrust flattery these days, and we're still not completely convinced of the necessity of our calling, but it was nice that he came in. And since he was a member of the bishopric, it's an indication that even if no one else cares, at least God and the bishopric do.
Today I'm thankful for good discussion in Relief Society, the member of the bishopric coming to thank us, interesting (at least to us) language questions, the refreshingness of popsicles, and kitties eating their food.