Soul Shells (魂弾, Tamatama, lit. 'Soul Bullet') are items in killer7, identified as bullets that have taken lives. They are collected throughout the course of a mission and then used as fare to cross the Vinculum Gate and progress towards the target. Soul Shells are encountered in every single mission except Target02: Cloudman and Target04: Alter Ego.
From a gameplay point of view, Soul Shells compel the player to explore a level thoroughly rather than rush straight through to the end; if he reaches the Vinculum Gate before acquiring all the necessary Soul Shells, he will have to backtrack to retrieve the rest. Many Soul Shells are encountered simply by entering every room and traveling down all possible paths, while others are the reward for solving various puzzles.
Despite what could be an emotionally heavy significance given what they are, most Soul Shells appear without any explanation. The critical exception occurs towards the end of the game, when collecting the Soul Shells scattered throughout the Union Hotel triggers Garcian's memories of having killed in turn each of the members of the original Smith Syndicate; the bullets and blood are still there decades later.