Bones
Bones has been hanging around Charney Chiropractic since 1997. Since he doesn't get paid, he doesn't do much and there's no telling if he'll decide to show up for work or not.
However, Bones will always show up in some kind of costume right around Halloween, just because it's his favorite holiday. Other than that, we hardly ever see him since his only interest is, well, as we said before – Halloween.