The duodenum is the first part of the small intestine...The pancreas and gallbladder secrete their digestive juices into the duodenum to help the digestive process and allow to efficiently absorb nutrients.
Doc Duodenum is a giant living-breathing organ. The Doc used to be part of an alien inside, but tired of his mundane life, he took off on his own which was not good news for the alien. He now resides on the Living Intestinal planet...a planet that is basically a gargantuan intestinal tract built in his own vision of perfection. Not too pleasant to say the least.
The Doc is not a very pleasant fellow. He's a giant bouncy-slimy-squelching detached duodenum after all! He likes nothing better than to spurt bile and acid out of his head and glands. His sole purpose in life is to digest things, so being found on his planet wouldn't be a very good idea, as he'd probably try to have you for dinner.
However despite the corrosive and deadly bile that oozes from his body, Doc Duodenum is quite a tidy thing; always fussing over the condition of his intestinal home, and making sure everything is in working order.