I don't get it
The backstory involves Yukari knowing Yuyuko when she was human, before she sacrificed herself to seal a youkai tree. Yukari wants to keep her in her "cage", i.e. unable to remember her previous life, and remain ignorant of the world, much like the caged bird. She loves that she can give Yuyuko her new "life", but feels that she's doing an awful thing by "caging" her in ignorance, so she is troubled by guilt.
