Geist

Geists are a species of Grimm that resemble a ghost and are capable of "possessing" inanimate objects.

The species made its first appearance in the "White" Trailer, as an Arma Gigas that engaged in a battle with Weiss Schnee.

Appearance
A Geist's base form is a black figure that has bone-like spikes on its back and ribs on its upper body, with long, skeleton-like forearms and sharp fingernails. Due to its ghostly appearance, it lacks a lower body, instead floating through the air while leaving behind a black smoke trail. Its face consists of Grimm markings on a mask-like bone plate with a single eye.

Powers and Abilities
A Geist is a possession-type Grimm, capable of controlling any object it possesses as extensions of its own body. Should it lose any of its limbs, it can repair the chink in its armor almost immediately by simply possessing another object in the vicinity. Its only real vulnerability is its "face", the central segment from which it maintains control of the form as a whole. Once its host object is destroyed or severely disabled, it abandons the body, revealing its base form. The Geist has yet to demonstrate any combat abilities in this form, but it appears to be relatively frail; it was killed with a single shot from Crescent Rose.

Appearance
An Arma Gigas is a suit of armor that has been possessed by a Geist. The "White" Trailer features one that takes on the appearance of a medieval suit of armor with clawed finger tips and pointed protrusions on its helmet, fingers and knees. It stands at least twice as tall as Weiss.

The Arma Gigas has a very different appearance in the manga chapters that explain the events of the "White" Trailer. In these chapters, its helmet is more rounded and has two separate openings, behind which a pair of glowing eyes are set. The rest of the armor also has a widely differing appearance, though both have a medieval aesthetic to them.

Powers and Abilities
The Arma Gigas displays the full mobility of a human, capable of running, jumping, punching and holding objects. The armor itself moves quickly for its size and is shown to be strong enough to wield a sword that is as long as it is tall with only one hand. The material of the armor makes it difficult to damage.

Appearance
A Petra Gigas is formed when a Geist possesses rocks. The one seen in "The Next Step" consists of gray boulders, which form a central body and four levitating limbs. Through the Geist’s influence, these boulders gain red vein-like markings. During its fight against Team RNJR, it loses one of its arms, which it replaces with a tree trunk.

Abilities and Powers
Through its stiff and slow movements, the Petra Gigas utilizes its enormous mass to deliver heavy attacks. The rocks that form its body are virtually immune to slashing attacks as well as lightning and fire Dust ammunition. Because the rocks are not physically connected to one another, it is possible to weaken the Petra Gigas by forcibly removing its limbs. However, it can possess nearby objects to replace its lost limbs, as seen in "The Next Step". In that episode, a Petra Gigas turns an old tree trunk into a new arm, but without an elbow-like joint, the tree arm proves to have less mobility than the rock arm that it replaces.

Trivia

 * Geist is a German word meaning "ghost", or depending on the context, "spirit" or "mind".
 * Petra Gigas means "stone giant" in Greek and Arma Gigas means "Weapon Giant" in Latin.
 * The Arma Gigas from the "White" Trailer is first mentioned to be possessed by a Grimm in Chapter 3 of the manga. However, it is described as being an artificial amalgamation of multiple specimens rather than a single Geist.