A Game Mechanics Developer is responsible for designing and implementing the interactive components of a video game. They conceptualize and build the rules and systems that govern the player's actions, the game's response to those actions, and the overall flow of the game. This includes elements such as character movement, combat systems, puzzle design, scoring systems, and player progression. They work closely with other team members like artists, animators, and writers to ensure that the mechanics support the overall aesthetic and narrative of the game. They also troubleshoot and refine mechanics based on testing feedback.