Rubber Duckie

Rubber Duckie is a toy lady duck that Ernie loves and often annoys Luigi with. She can talk to Ernie, which surprises the others. She speaks in a valley girl accent. Ernie and Duckie have actually had sex multiple times throughout the series.

Trivia
She has spoken twice, in Ernie's Birthday she said "Today's your birthday Ernie!" and in Shut Up And Dance With Me she said "I touch you while you sleep" and "Hungry for yo mom's ass."

Inspite being a rubber duck, there are several instances where Duckie is shown to possibly be alive:

In Come On Motherfuckers Come On she  cries at the thought of sharing Ernie with Daisy.

In The Go Kart Race when Ernie falls out the go kart, Duckie actually drives the kart and wins.

In Ernie The Magician, when Ernie said "OH SEXY ASSISTANT!", she turned around to Ernie.

In Truth Or Dare Duckie apparently walks all the way to Bert and Ernie's apartment.

In Equestria Lottery when Ernie asks Duckie what she would do with $1 Billion, she imagines shooting him with a gun.

In The Package when Duckie was "stealing" the package, she jumped for it and immediately took off her clothes.