Actually my thoughts is:
1. Owner met the demonlord (who appear briefly at the end of chapter 32) in a really weakened state (to a point she fit in a bucket) before she met jahy.
2. Owner knows jahy from the demonlord, thus invite her so she can keep jahy close to her.
3. The only way to restore the demonlord is by using gem.
something like that.
Oh and i love that jahy cry not because she lost her gem, but because she thought the owner betrays her.