@Miliam Yukari made a promise to live forever with yuyuko (that's the memories she refers to when she says "the one still bound by those memories is me"), I think it has to do with how ghosts form from having unfinished business or are still atached to something and once they take care of that they dissapear ( in Yuyuko's case it's the memories of her suicide), so everytime she remembers (lives again) Yukari hides her (let's her die) instead of erasing the memories (which would cause yuyuko to die permanently, which is why Yuyuko at some point says "just let me vanish"). Yukari is keeping the promise she made and she's mad at Yuyuko for breaking it.
Me thinks...