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Forretress is a large, spherical Pokémon with no visible limbs. The upper and lower halves of its body are capped by bumpy steel plates. The outer plates have zigzagging ridges that open along its back and front, revealing its smooth, red inner shell. The inner shell is punctuated by four cylindrical spikes, each tapering to an open point. Between the spikes, the inner shell opens along a straight line to reveal Forretress's eyes, which are round with large pupils. Whatever else lies inside its shell is unknown.
Forretress can defend itself from enemies by shooting out pieces of its outer shell. It opens its shell to capture prey, but does so quickly that its interior body cannot be seen. It lives in forests, attaching itself immovably to tree trunks.