Wolf, the main character of Sekiro: Shadows Die Twice, is equipped with a special prosthetic tool that can be modified in various ways. The player is able to use three tools at a time.
Each prosthetic tool can be upgraded. You unlock this option after defeating Gyobu Oniwa in Ashina Castle Gate Fortress [Ashina Outskirts]. Defeat this boss, and you will receive Mechanical Barrel. Take it to the Sculptor - this will unlock the prosthetic tool upgrade tree.
Speak with the Sculptor to access the prosthetic tool upgrade tree. Every upgrade has requirements:
- You need to own the previous upgrade.
- Pay a certain amount of sen and own specific crafting materials (collected through the game).
- Own other prosthetic tools - for example, to upgrade Shinobi Firecracker, you need to unlock Divine Abduction prosthetic tool first.
- Remember, better upgrades require rare crafting materials - look for them carefully in the later sections of the game (you won't get them from starting enemies).
After crafting an upgrade, you need to replace the current tool with it. Open the inventory window and, e.g. replace Loaded Shuriken with Spinning Shuriken.