From Brotato Wiki
You buy items using Max HP instead of materials
How to Unlock
Win a run in Danger 5
Unlocks Obliterator if you win a game with Demon:
For the Demon, items cost Max HP instead of Materials. At the end of each wave, half your materials are converted to Max HP at the rate of 13 Materials = 1 Max HP. Items in the shop have their price changed into Max HP at the rate of 20 Materials = 1 Max HP Rounded up. This means that if an item costs 1-20 Materials, it will cost 1 HP. 21-40 Materials will cost 2 HP and so on.
Coupon works like normal on Demon. It reduces the cost of the items, and if this pushes the item below a 20 material threshold, the Max HP cost goes down by 1. So this is why a Torch costs 1 HP in the first shop, but Ghost Axe cost 2 HP.
During the early shops, items that grant an increase to your Max HP can often be bought for less HP than they grant. Take for example Broken Mouth, Cake, or Acid. This can give Demon free Max HP which also means free money during the early game. The items have downsides, but the free Max HP is more than often worth it in the early game.
The Demon is also able to use their starting 10 HP to shop with, and this allows the demon to make out like a bandit from the first few shops, while items are cheap. Experienced players sometimes play Demon with 0-1 HP for the first 6-9 waves.
If Demon buys Handcuffs, the cost of buying Handcuffs will be deducted from their HP first, then their HP will be capped at the lower value.