Madpat

Madpat is the evil counterpart of MatPat who appears rarely in videos. His biggest appearance was in "The Secret Life of Markiplier" released on September 2nd 2018.

Origins of his name are unclear but could possibly allude to Inspector Gadget. His name was subtly confirmed by MatPat with the web page "hasmatpatgonemad.com" Mad. Later confirmed in Gate 6 of the GTARG.

Quotes
"Yes. We're tired of living in your shadow MatPat. It's time that we take back control" -MadPat

"Nothing is as it seems." - MadPat

Theories on MadPat
The direct reference to MatPat shows that he knows he exists and MatPat looking at him shows this is mutual. Also MatPat hides a smile when he sees MadPat. It seems MadPat and WarfPat are working together because of there shared line.

The line "take back control" and "Nothing is as it seems"  tells art of a story. It is possible a story of two beings forming a channel together. Then one overthrowing the other. MadPat could be the mastermind behind all of the dark theories.

Some people think he hides in a Freddy suit.