Coomer refers to a Wojak character described as a person with severe masturbation addiction similar to that of the Cumbrain. Unline similar 4chan Wojak characters such as 30-Year-Old Boomer and Zoomer, Coomer's description includes multiple parody elements which could not be applied to a real person due to humorous exaggeration.
https://knowyourmeme.com/memes/coomer