Much of the Slitterhead gameplay revolves around Rarities, gifted humans who can achieve higher attunement with the mysterious Night Owl and maintain their consciousness while being possessed, which allows them to use some unique fighting styles and skills that can make your life easier against all sorts of opponents.

Unlock new Rarities is also a necessity to proceed in the main campaign, as you will need the help of specific characters to continue Night Owl's war against Slitterheads. The first time this happens is right after starting to investigate a religious cult that seems to be bringing together monsters and humans, and you will need Edo's help to proceed in this investigation.

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Once you are notified of needing a new Rarity to proceed through the story, play the Parasite Mission 2nd round mission again. At the start of the mission, proceed normally until you reach the main street. Walk around until you hear two NPCs talking about a stray dog and a homeless man. Continue past these two characters, and you will find the stray dog, who is the same one you have met at the very start of the game. Possess it, follow the scent like you did at the beginning of the game, and you will eventually reach the homeless man named Edo. Possess the character, defeat the Slitterhead, and he will join Night Owl in his quest. Quit the mission, and the Hidden Cave RE 2nd Loop mission will be unlocked.