Because some people say that Haise is just a new character. I just want information because I'm confused.
Spoilers for those that haven't seen the show.
When Kaneki was trying to live as a human his hair was black, then when he decided to cross into the world of ghouls, his hair went white. While living amonst the ghouls his hair remained white. Following this, at the end of the series his hair starts to turn black again as he crosses back to the side of the humans. Now, as Haise Sasaki, who lives as something between humans and ghouls, his hair is white with black in it. There is clear visual symoblism that proves they are the same character. Not to mention plot I don't want to spoil.
I'm assuming you are talking about Tokyo Ghoul. In the manga (at the place where season 2 of the anime ended), Arima kills Kaneki and stabs him in the eye. Kaneki does "die", but as tha manga continues,
POSSIBLE SPOILERS FOR SEASON 3!
Kaneki "comes back" as Haise Sasaki with little to no memory, so techinically, he didn't comepletely die. In the anime, it never shows Kaneki's fight with Arima at the end of season 2, but if there is going to be a season 3 which continues from where season 2 left off, then we can safetly assume that Kaneki did get killed by Arima.