Mainframe is a malevolent monster of a Decepticon. Rather than possessing a conventional alternate mode, Mainframe changes shape by draining the energy from any mechanism, living or unliving. He can then duplicate that mechanism's form to the smallest detail.
He also possesses the ability to generate numerous drone units by duplicating his own body structure, although apparently even this degree of authority is insufficient for the megalomaniacal Mainframe - his thirst for power may be as unquenchable as Megatron's or Starscream's.