Rensa

Rensa is the name given to the cyborg which was created for the emergency event when all Level 5 Espers would rebel against Academy City, in which case she would suppress them by using their abilities or copying other Espers' abilities. Being only a cyborg body, small parts of a person's brain can be inserted into the cyborg in order for them to "pilot" it. There are 40 recorded individuals who where willing to have their brains reduced to a very small portion in order to control Rensa in turns. Only #28 and #29 amongst them were shown in action. (Source: Toaru Majutsu no Index Wiki)