Yvonne is a bandage-wrapped mass that burst from Salad Fingers' stomach in episode 9. It is covered in a black goo, presumably Salad Fingers' deteriorated insides.
Salad Fingers treats this mass as a baby and names it "Yvonne." Salad Fingers then states,"You've got your mother's eyes. And she was a fair maiden too." This may indicate that before the Great War, Salad Fingers may have had a child. At the end of the episode, he insists that Yvonne go and live with his "Aunty Bainbridge" (the Yellow Child) while he is in an unhealthy condition.
He apparently forgets that Yvonne is his "child," and ends up using it as a washrag to clean the Yellow Child's windows, which does nothing but turn the windows black.
It is possible that Yvonne is actually the child that Salad Fingers burned in the oven during episode 2 and he ate him mistaking him for the fish which he had also cooked. This may also explain why it is a charcoal mass.
However, it could also be the black 'hair' worm he ate in the previous episode, which had fed inside him, causing him to grow sickly from lack of nutrition due to the parasitic worm.
Edit: As Salad Fingers is seen eating a sandwich in the exact same shape as "Yvonne" - after he wipes all the goo onto the window - it is very likely it was a bad sandwich simply covered in whatever black came from Salad Finger's stomach.