“Hide lets Kaneki eat his face to get back his strength. While it appeared at first that Hide had died in the process, he reappeared later as Scarecrow, an ally of Kaneki and the ghoul hunters.” “Kaneki had eaten the lower part of Hide’s face and that’s why his voice box was also eaten.
In the manga kaneki ate part of hide to survive which he didn’t do in the anime, because it wasn’t shown so I’m not sure in the anime how that happened but it’s probably the same thing. Basically kaneki ate his face.
He is the same person as ‘Ken Kaneki’. It’s just that after his defeat to Kishou Arima during the Owl Extermination, he had Amnesia. He was then brought up as a ‘Ghoul Investigator’, and trained under Arima.