Sekiro Shadows Die Twice - Memorial Mob Trophy Guide / Achievement Guide - Encountered the Memorial Mob (Shop Merchant / Store Vendor).

This can be done right after the prologue when you reach the region Ashina Outskirts. After the idol figure checkpoint called "Outskirts Wall - Gate Path", get past the samurai miniboss "General Naomori Kawarada". Then follow the path down the left. There's a big mountain on the left side of the area. With your grapple tool you can jump from tree branch to tree branch until you reach the top. On top of that mountain is a tent with a merchant inside. This is quite hidden as you'd never have reason to go there normally. It's just before a guy shoots at you with a cannon from a wall.