New Cortana model, same likeness, modeled after Halsy again, only like Cortana said, *its not her*, which opens up discussion about AI being alive and having souls.
This Cortana model gets renamed by Chief, who treats her as Cortana's twin sister. Game ends with Chief officially having a funeral for Cortana, talks of AI being sentient life begin, and having rights.
Cortana's new twin sister (whatever she is named) is his new AI for the rest of the series, Cortana is treated as a dead war hero and still reffered to through out the rest of the series, the twin sister tries hard to live up to the service record Cortana left behind, and meet and exceed those expectations.