Rabbert Klein is one of Dr. Krieger's test subjects. In the ISIS applied research laboratories, Rabbert was a successful test subject for a mind control microchip that was previously inserted into Sterling Archer's brain.
After the chip is removed from Rabbert, it is stapled into Len Trexler's bald head. Trexler then asks if he can have Rabbert Klein and the lettuce, because "they are brothers."
Sterling is responsible for providing Rabbert's name, which is a homage to the comedian Robert Klein. The name is appropriate because Klein's comedy often involved an "I can't stop my leg" routine. When Rabbert is introduced, Krieger is forcing his leg to move with the mind control device.
It is unknown if Rabbert is still alive or not, and with Len Trexler.