Corporal Matthieu is an NPC in Unturned 3.
He can be found at Jourdan's Fort, on the ground floor, in a small room. This NPC gives multiple quests.
High Polymer and Tempered Steel: (This Quest can only be started after completing a quest from Sergeant Boone.) With the kit obtained from Sergeant Boone, Corporal Matthieu can upgrade a Hecate for you, but you will need to get one first.
Heavy Weapons: (This Quest can only be started after completing enough Supply Quests.) Corporal Matthieu wants to start making actual weapons, and asks you to find the blueprints for them. He also asks you to find the necessary tools to work on them.
Supply Quests: Corporal Matthieu asks you to retrieve supplies for him. The supplies he will ask for are as listed: