This nameless character (often referred to as Stitch-Head) appears in Episode 4, Cage. It resembles an embryo.
Stitch-Head doesn't appear to know any human language, but does babble and make babyish noises. It seems to have an attraction to Salad Fingers. In episode four, it lures Salad Fingers with a rusty tap, captures Salad Fingers with a bear-trap and locks him in a cage, proposes with a ring, and gets mad when Salad Fingers rejects its passes. The episode ends with Salad Fingers pulling on a random rope from inside the cage that brings down a red curtain, preventing the creature from seeing him; when the curtain rises, Salad Fingers is gone. After realizing that Salad Fingers has somehow escaped, Stitch-Head sits down and begins to cry.
He isn't mentioned or seen again for the rest of the series.
Stitch-Head's stitches imply that someone (possibly Dr. Papanak) with coherent thought put him together (stitches are precise and require attention). He is also obviously mutated. What his fascination is with Salad Fingers is unknown, as Salad Fingers clearly dislikes the mutant, possibly meaning Stitch-Head is the closest thing to the main antagonist of the series.
Stitch-Head also got ahold of a large cage he used for Salad Fingers. How many other people it's put in the cage and how he manages to catch them all is unknown, as well as how he knew Salad Fingers had an obsession with rust and water taps. One possible answer is that Salad Fingers never had an obsession with water taps, and he made up his obsession on the spot due to his insanity.