ISO Didact was a 12 year old forerunner who had the Ur-Didact's consciousness imprinted in his mind. In time the consciousness overrode the original mind and he effectively be and the new Didact. Ur-Didact is the one in halo 4. He hates humans because of his war with them while his other half loves them because his best friends were human. Ur is German for original, ISO is Latin I think for new. Something like that.