When Donnie is Krangified, he has no free-will of his own and has to follow the orders of Krang Prime and his siblings. Sometimes, his real self can come through, but it is difficult. It is possible to free him of the Krangification, but only by pulling the main part of Krang flesh that is controlling him off. The part controlling Donnie is the small extra eye between his normal eyes. If pulled off, the rest of the Krang flesh on his body will fall off him and he'll become himself again.