My take on MEF is a little different...
He was once just a normal guy who was mortally wounded on the battlefield, and MAA constructed a life support system for him, which was permanently fused with his body.
This life support system is controlled by a highly advanced CPU, which is the knob on top of his head. The CPU is directly wired to MEF's brain and the harness, and it monitors his vital functions and also aids him in battle by releasing chemicals into his bloodstream.
Now this is where things in my interpetation get a little far out...MEF actually only has one face, his human face. His face doesn't turn or switch to any other. The robot face and the monster face don't actually exist, but of course as a toy or when watching a cartoon how do we know what personality is in charge? So these 'faces' are just representations of his different personalities.
The human face represents MEF's normal personality. He loves acting, and is often found performing plays. He is a noble and loyal person, who is indebted to MAA for saving his life. The human form is MEF's dominant personality, and he secretly hides from the others the dreadful secret of his robot personality. He is saddened that he has to battle for dominance over his own mind, and is always very remorseful when his personality slips into the monster personality.
The robot face (or personality as I prefer to see it) manifested when this highly advanced CPU developed an awareness of itself. The robot face is when the CPU manages to surpress MEF's humanity in favour of itself. It isn't evil, but like all life forms it seeks to establish superiority over another life form, and in this case, that's MEF's human personality.
In robot form, MEF leaves no trace of the human personality, and MEF will act in a cold, machine-like, robotic manner.
It cannot kill of MEF's physical form, as it relies on MEF's body to create the energy needed for it to function.
The robot personality is usually surpressed, but sometimes it manages to take hold, especially when MEF sleeps. When he awakes, the robot personality will be present.
It often takes a supreme effort of will for his human personality to regain control.
The monster face (or again, personality) is the reperesentation of MEF's dark side. This personality emerged as MEF's humanity struggled to supress the robot personality.
This personality feels cursed, in that the human is permanently bonded to the machine, and it feels trapped in this life support system. Secretly, it hates MAA for making him live this way, even if of course it was the only way to save MEF's life.
The monster personality, is a dark, brooding, unpredictable personality. It hates all things technological and it can be very violent, sometimes flying into a dark rage. This rage causes a spike in MEF's testosterone levels, and as a result, the CPU, thinking that MEF is in battle, will release endorphins and performance-enhancing drugs into his bloodstream, making him superhumanly strong and extremely vicious.
While an asset in battle, it can also cause problems since the monster personality can be manipulated. His supreme anger sometimes causes him to lash out blindly, even at friends.
Unlike the robot personality, there is no definite switch of personalities...his monster personality emerges as the human personality slips into a state depression or anger. MEF's monster personality will be dominant for as long as the anger inside him rages on.
Once calm, the monster personality will gradually recede, and MEF's human personality comes to emerge once more.
MEF will sometimes willingly and knowingly (but always regretably) bring on this monster personality to aid him and his friends in battle.