Ernest is a conjoined twin who is attached to Luigi.
Ernest was first mentioned in the Corruption Stockpile 3 stream. Vinny was choosing a character tag along with Baby Mario when he scrolled to Luigi. Under Luigi's nose looked like a face, not to mention the stretched skin at the bottom. After seeing this, Vinny said in his DiC Mario voice, "Luigi, when you were inside our mother, we had anotha brotha, his name was Ernest. When you came out, you took him with you, but not in the right way, Luigi." Those words are what created Ernest.