The girl, Yoshino Kogawara, was 19. Her friend, a 20 year old, sitting directly behind her, is in critical condition with a head injury as well.
Here's a pic of the wheel assembly from where it detached. That happened on the way down the hill towards the helix, right where the coaster gets most of its speed.
It was almost to the end of the ride, at that valley just after the upwards helix.
This graph shows how, and where the injuries happened. Pretty obvious.
Fortunately, she was killed instantly. There was no lying there in pain. Very sad.