Who was Pip in the Twilight Zone episode "A Nice Place to Visit"?

I know of no canon answer, and you're not going to get one now. However, it's likely the name is meant to reference a term which isn't really widely used now but was pretty common when I was younger. "Pip", or "Pipperoo" was used to reference someone who was really good at something. "Look at that little pip!" "Lynn Swann? He's a pip!" My dad used that term frequently.

Most likely coincidental, according to dictionary.reference.com it also has the following definitions:

verb (transitive) ( Brit, slang) pips, pipping, pipped
1. to wound or kill, esp with a gun

noun
1. one of the spots on dice, playing cards, or dominoes.

As for why he hasn't eaten in several centuries, my guess is that dead people don't need to eat. It was probably inserted into dialogue to make it obvious the characters weren't alive. TV back then had a habit of being overly obvious about plot lines.

The "who"... Now that's open to interpretation. My guess would be a minion, because the devil himself just couldn't give one person that much attention. Especially a low-level crook.

It's also quite likely a reference to an extremely obscure typo. A few ancient Bible manuscripts transcribed by hand wrote the number of the beast - the devil - as 616 (in Greek) instead of the more common 666. The Arabic numerals for 616 can look like PIP in a mirror.

https://en.wikipedia.org/wiki/Number_of_the_Beast

(this is in addition to, not a replacement for, Johnny Bones answer about the more current slang meanings of the word.)