Good Robotnik is a character that appears in Sonic Underground. He is an alternate universe version of Dr. Robotnik.
In an alternate Mobius where the Sonic Underground (Sonic, Sonia and Manic) are a evil rulers named "Royal 'Corrupt' Hedgehogs", Robotnik is the leader of the Resistance trying to take back the kingdom for the good of the people. At first, the "real" Sonic the Hedgehog obviously confused the good Robotnik for the real "bad" one, and made critical remarks of him. After a while, however, Sonic and the good Robotnik teamed up to defeat the Corrupt Hedgehog regime. He is very nice and congenial to the other Hedgehogs' good counterparts. Sonic Underground in turn showed the same respect back, though Sonic did once refer to him as "Robuttnik" by mistake, admitting it was a tough habit to break.