Ghost of Tsushima: Swordsmith
Ghost of Tsushima's Swordsmith is an NPC that can improve your sword and tanto, increasing the strength of your melee attacks and the speed of your assassinations. You use materials as currency, like Supplies, Iron, Steel, and Gold.
Swordsmith: The katana of clan Sakai... An exquisite weapon.
Jin: You recognize it on sight?
Swordsmith: For me, every blade is unique. As recognizable as a person. Lord Sakai. Bring the materials I need, and I would be honored to improve such a fine sword.
Upgrading Sakai Katana
You need Supplies, Iron, Steel, Gold to upgrade the katana. You can find a lot of them from Mongol Territories, always loot the place after defeating the enemies. Plus, you will get crafting rewards from liberating these areas.
The Sakai Katana has six upgrades, with the damage maxed out at the final upgrade.
Upgrading Sakai Tanto
The Sakai Tanto has similar upgrade materials as the katana. Hence, it is advised to always upgrade first the katana, as most of the time you will be fighting enemies head-on, instead of doing stealth. Also, the duels, leaders, and minibosses have to be fought upfront, so a high damage katana is always better than a tanto.
The tanto has two upgrades, with the speed and stealth maxing out when fully upgraded.