Shell / Benü

A shell is a type of walking tank. It uses a live human brain to control it. These brains were mostly taken from convicts or volunteers, not that there were many volunteers.
The first shell was technically Setari Hokra, an Empresses daughter who had been paralyzed in a duel for heir of the throne. Distraught, the empress ordered scientists to fix her, or make her a new body. After many (unethical) tests, the first fully mechanical body was created. It didn't use electrical motors, and instead opted for Lerite ones, designed specifically for the purpose. The whole set up was not exactly a long term option, but it would give the princess back freedom of movement. It was sort of a hybrid between golem, and cyborg, all that remained of Setari's organic body was her brain and her bone marrow. 
Once the brain is removed, it is placed in a "brain cage" which allows the brain to communicate with the body, and also protects and maintains it. These became more sophisticated over time. The first brain cage was never replicated, due to all the research being destroyed by angry test subjects. But other scientists saw how useful the technology could be, and created new ones, using new methods that were only now becoming available.
Modern brain cages can be used to control the brain within them, and train or brain wash them until they are suited for their purpose. Some even had remote explosives, and could trigger the pain center of the brains in case they became insubordinate.
Shells were given immense firepower, and armour that outmatched most other vehicles. Combined with their intelligence and command over their bodies, it made them a formidable, and terrifying opponent in war. Due to stressful procedure of preparing a brain to be used with a shell, they often obtained personality traits or disorders, which translated into their fighting style or disposition to combat and war.
For example, some shells crave attention, attempting to talk to people constantly, dancing across the battlefield, trying to prove themselves deserving of fame.
Some become mute, though this may be caused by damage to the speech center of the brain, instead of being a choice.
Some lose all empathy, only fearing for themselves. These ones are difficult to control, and they sometimes accidentally kill fellow soldiers, or civillians.
Some have an immense fascination with certain things, like fire or explosions, death, food, or shiny objects.
it's rare that a Shell isn't insane in some way. In an effort to stop this problem, some brains are given lobotomies prior to being placed in brain cages. It removes personality and makes them easier to control, but also removes their passion for anything, making them less effective in war. Another experiment involved using childrens brains (orphans), because of their ability to adapt better than an adults. It did work, but they turned out to retain their personalities before having their brain removed, acting as if nothing had happened, it was not good for morale, not to mention highly immoral. The practice was banned after the first batch.
The lifetime of brains placed in brain cages are much shorter than a normal brains, and their inherent personality traits and disorders get worse over time. Interestingly, princess Setari was able to live for almost 130 years, and had an undamaged personality with no disorders (until she had her brain transfered to a new body).

Benü come in several classes: