Wheatley, glados or Navi from Zelda. Although if it's Navi the only sounds she can have are "HEY! LISTEN! HEY! WATCH OUT! HEY! HEY! LISTEN!" Or perhaps a scyther just repeating random parts of his name and everyone just somehow understands it. Although if I could have two ghosts at the same time I'd have the space core and fact sphere from portal 2.