What character is known best by the title "Gatsby's boarder"?

Ewing Klipspringer was known as "the boarder". He hung out at Gatsby's parties and enjoyed unlimited use of Gatsby's home and belongings. He was for all intents and purposes..... what is known as a leech.

A man named Klipspringer was there so often and so long that he became known as “the boarder.”— I doubt if he had any other home.