Sugar is Zone 0’s Gaurdian. The reason why no one else is in Zone 0 is because she ate them all (the Elsen are made of sugar, and sugar is addicting…). When word reached the Queen, the Queen had her locked up where surely no one would find her. This is the reason why Zacharie said “It’s better this way,” after Batter kills her. She would surely not be able to eat any others if she were dead.
The Toad King was the Guardian of Zone 0. He was corrupted and mean-spirited though, and treated his ‘subjects’, the inhabitants, badly. In the end, the was slain by Zacharie for it, with some help from Sugar/Sucre. Zacharie then made the Toad mask out of the Toad King’s remains, and Sugar/Sucre became the new guardian of Zone 0.