After beating Zone 1, you can come back and make your way into its Purified form.

Keep following the tracks until this part after riding the Pedalo.



Inside it, there's a chest with an item called 'The Grand Finale'. Take it.

Our next stop is Zone 0. You can talk to Zacharie if you want, but to find 'Sugar' we'll have to enter to the first door in the middle.



If we have any of the three Purified Zones' Grand Items (In this case we have 'The Grand Finale'), we'll see a staircase inside that building. From there it's a straight way to her.

Here's the personification of its homonymous substance. Sugar is a tough boss with a max HP of 12000 and four companions.

She's weak to Poison despite the Wide Angle (or the Eye item) displaying a text that states that she has no weaknesses.

You can aim to kill her and ignore the damage of all her dummies, or kill the dummies first and then take care of her. It's your choice. Keep in mind that 'Sugar' has been the hardest boss of OFF since always, at least until 'Cob' appeared in this new edition of the game.

If you beat her, you'll obtain the 'Grand Chocolatier' which you'll need if you want to see the True Ending.