[frubam]

Originally Posted by Tempest35

You may have forgotten one more option, Xellos:

Spoiler for Option 3:

Sacrifice

Our Yuuko is much to nice and innocently (if not a bit sensually) mischevious to be actually sealed up out of fear of being evil.

Spoiler:

But can you truly say that with certainty? She HAS lost all of her memories. Her current mischievous nature could simply be the result of being lonely + being a ghost. Who knows what her original personality is like.

Perhaps "self-sacrifice" would be the appropriate 3rd option. Something was happening and she decided to do something herself when she ended up down there. That stone with her name on it would mean someone meant something enough to Yuuko to make such a thing, maybe out of thanks that she did whatever it was to stop "something" from happening.