Shells function basically as character classes or armor sets in Mortal Shell, where each has specific abilities and attributes unique to them that are all upgraded separately. Finding your first shell location will award the trophy 
Flesh and Instinct , and finding all 4 shells will award the trophy 
You Seem Different…
They are all found in the initial area of Fallgrim immediately after beating the tutorial boss, and can all be obtained right away without doing anything prior.
1. Harros, the Vassal
After the tutorial, you are in a forest area, named Fallgrim. Continue forward on the path until you reach the split in the road, then go to the left to find this first shell.
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2. Solomon, the Scholar
In Fallgrim Tower facing Sester Genessa, go out the door to the right and straight forward into the cave. In here you can fight the first miniboss Grisha. After it is defeated, go down the slope at the back and crawl through the hole to find another enemy. After it is defeated you can pick up the shell by the torch on the ground.
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3. Tiel, the Acolyte
From Fallgrim Tower, facing Sester Genessa, go left and crawl out the hole, then down and through the graveyard area out the other side. At the cooking pot that some Brigands will be sitting around, look to the right to see some wood blocking a hole, which you can break then crawl through to the other side. Here you fight a miniboss named Ven Noctivagu. Once it is dead, you can interact with the Shell on the ground to the left.
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4. Eredrim, the Venerable
From the room where you fought Grisha then went to the Solomon Shell, go to the right and crawl through the hole. Follow the left wall all the way to the end until you reach the area with all the stone slabs and large rectangular blocks leading to a tower. Go down the path to the stone area, then you can find this shell leaned against a fallen block on the left before the set of stairs.
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That’s all 4 Shell Locations in Mortal Shell.
More Mortal Shell Guides:
- Mortal Shell – All Item Locations
- Mortal Shell – All Secret Lore Locations
- Mortal Shell – All Weapon / Equipment Upgrade Material Locations
- Mortal Shell – Boss Guide (All Bosses, No Shell/No Damage)
- Mortal Shell – Trophy Guide & Roadmap
Found all 4 shells and got no achievement.