Tsubaki was a fusion of a human and a Festum, and as a core, she governs all functions from life support to defense of the island.
Also, she was trying to teach the Festum about the human emotion of grief.
In Exodus,
In response to Emery's wish, Orihime brought a new Fafner core to the island. She also orders Kazuki and Soushi to leave the island and fight as soon as she wakes up. Seri is puzzled by Orihime's behavior, which is very different from Tsubaki's... Orihime has inherited all of Tsubaki's memories and consciousness, but her personality and her feelings are a of completely different person.
