Video games rarely make all of their content available upfront. It's common for video games to keep rare content tucked away, prompting players to go out of their way if they want to unlock them. Some games go further and include unlockables that the player doesn't even know exist. This rewards players who explore and experiment, building a community among players trying to solve these secrets.
Source：https://www.cbr.com/rare-secret-video-game-characters/
