An emissary is a druid advisor to werewolf Packs. The Emissary's job is to give advice, serve as a confidante and keep the wereanimals connected to humanity. Traditionally, only the alpha in the pack knows who's the emissary for their pack.
The relationship between the druids and werewolves began in ancient times. Lycaon and his disciples had become the first werewolves. It was said the celtic druids knew how to shapeshift. So Lycaon's kin sought out the druids in the hopes they could make them human again. The druids couldn't reverse the curse, but they did teach Lycaon how to shift back and forth.
Ever since then, druids have served as wise counsel for the werewolves.
Two emissaries that appear on Teen Wolf are Alan Deaton and his sister Marin Morrell.
Deaton, a veterinarian for Beacon Hills is a known healer, as well as an advisor, for canine werecreatures, using his animal clinic as a base for the supernatural population on many occassions. He served as the emissary to the revered Alpha Talia Hale back in the day, and now serves as one and a mentor for Scott's Pack.
Morrell, a guidance counselor, served as the emissary for the Alpha, Deucalion, even after he began a campaign of bloodshed and terror forming the Alpha Pack. Morrell, however, maintained her obligation to maintaining balance and compensation, by actively working against them, discretely hiring a mercenary to hinder their actions.