As this picture shows us, Chiaki seems to be the center of all of her classmates.
Additionally in all of the end cards, she is in every single one, one way or another.
I think this represents how Chiaki will never die, all the happy memories the class has made together gave her purpose other than her talent. As long as these memories exist or have existed, she will never die.
She felt that she was lost and her talent was the only thing defining her, however when she saw everyone, people like her that find worth and happiness in this way, she found something within herself.
When she saw that things were going to shit and everyone was being consumed by despair, she made an AI of herself and gave it to Chisa.
Along with all the memories that she shared with them all.
Chisa probably had to run away when all of this was going down and that’s when the future foundation had started to flourish, using Chiaki as a source to help them.
And when they were making the Neo World Program, she was desperate to be there. At any cost, so she could atleast see how they once were, just one last time. She missed her friends.
I think what happened to Chiaki probably broke everyone’s hearts, since she was probably the one (except for Chisa) always keeping them together and getting along.
And when Izuru killed her, they all fell into despair, encouraged by Junko of course.
This would also explain why Izuru is always far away from the rest of the ultimate despairs.
Because they saw (or knew) what he did and they would probably slaughter him on sight.
That’s all I have for Chiaki for now.
I’ll probably add to this later ^^
EDIT (ADDING TO THE ARGUMENT): The wonderful @theescapingspectre thought of this one, and I just had to share it, this is it word for word.
“You mentioned when Izuru killed Chiaki what if Hajime still existed and still remain in control and argued with “Izuru” in his mind to leave her out of the despair but instead Izuru gave her the choice to join him as part of Ultimate Despair, Chiaki obviously refused and got killed for her choice but not before doing the AI of herself and given it to Chisa. This broke Hajime’s spirit even further and it eventually disappeared but not before he shed a tear and became Izuru Kamakura completely.”