Fae
Nearest to the mortal world are the Fae: Summer, Winter, Wyld. You know of Summer and Winter, certainly, but the Wyldfae vastly outnumber both Summer and Winter. you are much more likely to encounter the Seelie and Unseelie. They enjoy meddling in the affairs of men. They find it a relaxing diversion from the normal routine of their long-lived existence.
Keeping some form of iron—the bane of Faerie kind—about you at all times is an excellent survival tactic. You will often hear the term cold iron, but any form of iron will protect you against Faerie kind. The profusion of iron throughout our world helps limit the degree of Fae involvement here.
By the unschooled, the yin and yang are considered two sides of a coin, opposites, competing forces in nature. Many philosophers, however, would note that the two are complementary, not competing, and in each exists a little of the other. Summer and Winter are like the yin and yang, moving in concert, each mirroring the other’s steps and actions.