There's a lot of characters who can't die.

Krile, for example, is in charge of the Eureka exploratory missions.

If you killed her, she would have to be rewritten in the existing story. They won't do that.

Alphinaud and Alisaie also can't die. They ran the Binding Coils of Bahamut content.

When you start taking apart what characters have vital roles in additional content, you start to see who can't die.