Dennis is a gun seller and a friend of Dr. Jennings.
In Sam Winchester's dream vision Dr. Jennings keeps walking, and goes into a store and heads for the gun counter. He asks to see a gun, and Dennis, who knows him as he is a doctor, gives him the one he asks for. When Dr. Jennings has it, he asks what kind of shells it uses, and Dennis shows him. Dennis then offers to take him up to the cabin with him when he goes that weekend to hunt turkey, but the doctor says no, guns freak me out. He starts loading the gun and Dennis tells him he can't load the gun on the premises, but the doctor replies "It's okay. It's all gonna be okay, Dennis." And then he shoots Dennis.
After this vision, Sam goes to the store where Dr. Jennings buys the gun, and then he pulls the alarm and leaves the store. Dr. Jennings walks up outside, then stops instead of going in, and walks away. Thanks to this, Dennis survived.