What happened to Ken Kaneki in Season 3?
In Season 3, he works for the CCG under the name Haise Sasaki until he regains his memory. In Season 4, he finally becomes the One-Eyed King and fights the Dragon, an evil form of himself, to save both the ghouls and the humans.
Why did they change Ken Kaneki?
It’s after being tortured that Kaneki undergoes his most iconic shift, accepting himself as a Ghoul as his hair turns white. Another involves his awakening as a Ghoul boosting his count of the fictional “RC cells.” Regardless, the change was a symbolic for Kaneki’s change as a person.