So, Gunko was indeed a princess, but we also know she was Imu's handmaiden, when Xebec confronted him, 56 years ago, and then she was a Devoted Blade at God Valley, 38 years ago. But even in the present, she hasn't aged at all. So why give someone who wasn't originally a World Noble, by birth, an Abyss Covenant? Why is she so special that she gets immortality unlike "true" World Nobles like Sommers and Garling?
And also, is this why Imu is pursuing Vivi, to make her like Gunko?