A downvoter is someone who uses a YTMND account mostly to downgrade the rankings of many sites by giving them the lowest possible score. This is known at YTMND as downvoting. Once a site has been downvoted, its overall rating suffers, making it difficult to restore and less likely to draw attention to potential viewers. What sets a downvoter apart from a normal user is how a downvoter almost always rates each site with , regardless of how good the site is.
It's believed downvoters can quickly downvote many sites by directly accessing the site's profile without looking at the YTMND itself. Max has taken measures to prevent or slow down downvoters by deleting their accounts. At one time, Max deleted 2,211 downvoter accounts, reversing over 250,000 votes. 
The opposite of a downvoter is an upvoter.