[ Ori doesn't know Vanitas or Aether on a personal level very well, and doesn't know what their circumstances are, but she does know that the decision to leave one's world to follow another into theirs is not a decision to be made lightly. And she also know that the one they've made means that Aether can continue on his journey to find his sister, and for that alone she can only be happy for them. ]

Mm. Wonderful. Then you'll have someone to share your burdens with! And to introduce your sister to, when you find her!

[ Not if - when. Just because she doesn't know him well, doesn't mean she can't hope for the best for him. And as someone who went on her own journey to find her sister, well... She has to root for him, of course. ]