Jinx is the marten that travels around with Zephyr inside his coat most often. He has his own special powers and gifts. He is Zephyr's best friend besides Valli and at the same time, his constant frienamie.
Jinx is able to teleport anywhere he wants, grow to be the size of a two story building or shrink to pencil size, he has dark magic that can do pretty much anything and he is immortal.
In full "demon" form, Jinx becomes more powerful by a tenfold, but since it's so hard on him to be in that form he doesn't assume it often.
Jinx is kind of like the missing part of Zephyr's personality. He is loud, talkative, can be rude, doesn't take s*** from anybody, boisterous, sarcastic and slightly condescending at times. But like Zephyr, he is fiercely loyal, has best interests at heart (most of the time) and he can be nice on occasion. He is extremely protective of Zephyr too. Jinx is adorable and he knows and loves it.
Not much is known about Jinx other than he is mostly a marten who is part of the "Zephyr Package" no matter what, has the biggest appetite in the tower, chatters to speak, lives inside a magic trench coat and will attack to kill if someone calls him a "Weasel" or "rodent" or anything of the sort.
Once Jay asked if Jinx had a "more to the story" kind of thing and Zephyr, being ever truthful, admitted there was. He didn't tell anybody what it was though. That's how Jinx wants it.
Jinx traveled to the spirit lords at one time to barter for Zephyr's soul. Once attaining it, Jay was able to bring him back.