Leeches are small, segmented worms. Most species of which live in or near water. While most leeches are predators, eating small organisms whole, the most well-known leeches feed on blood or similar body fluids.
These creatures were first seen in the season 3 episode, "Hard Drive Courage."
The leeches' bodies are entirely a dark gray color. Their facial features include two long purple eyeballs along with bizarre, wide mouths that include teeth.
Role in the episode
Like stated before, the two leeches first appeared in "Hard Drive Courage," where they were shown sucking blood out of Eustace in order to help cure his cold. When Eustace unveils to Muriel and Courage what he's dealt with, the two leeches are shown weirdly laughing.
The two leeches appear again later in the episode, when Courage makes a call from within the computer. Courage plans to reach through the phone and grab the jelly that was given to Eustace earlier in the episode. When Courage calls, Eustace answers the phone, and the leeches expect whoever is on the other line to be someone of interest. They exclaim, "Tell 'em we're busy."