She nodded simply, and mentally stumbled over how to explain the next part - and ultimately, which she was - without sounding like the monsters all Hollows were painted as. "Hueco Mundo is referred to as the world of Hollows, though if the Shinigami had their way, they would not have one. The truth is, most Hollows never even live to see its white sands and eternal night. Because..." First hurdle, get.

"...well, all Hollows of any significance start as a soul with an eroded Chain of Fate. This Chain ties the soul to something holding it back - its fetter, if you will. A loved one, a heartfelt trinket, whatever was held closest to the deceased's heart. If they aren't held back, it is not connected to anything. Over time, and especially if targeted by an existing Hollow, the Chain will erode. If it erodes completely, a hole forms and they become a Hollow. If a Shinigami gets there before hand, usually they are sent over. Shinigami hunt down and 'purify' these new Hollows."

Is that a slight bitter sting? Nel's not telling. Either way, she knows a lot about the workings of Hollows. "Sometimes they escape, and eat the souls of others. Many of the wandering Hollows find their way to Hueco Mundo as a solace. There they likely just get eaten by something older. I think the expression is... there is always a bigger fish?"