Weiss' Arma Gigas

Armored Knight (s the opponent Weiss Schnee fought in the "White" Trailer.

In Chapter 3 of the RWBY manga the armor is described as being a possession-type Grimm.

Appearance
The Armored Knight takes on the appearance of a medieval suit of armor, made of an unknown, but clearly durable space-gray metal. Its under armor is the same shade of gray, while the internal parts of the armor, namely the helmet's viewing area, take on a jet black color.

The armor stands at least twice as tall as Weiss, wielding a longsword which appears to be made of the same metal as its body, and is nearly the same height.

When destroyed by Weiss, its under armor and body begin to glow a bright white and dissipate before its armor shatters and disintegrates instantaneously into multiple ice shards upon impact with the ground.

Manga


The armor has a very different appearance in the manga. In contrast to the featureless blackness of the armor seen in the trailer, the armor that Weiss fights in the manga has a more rounded helmet and 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.

Like the one seen in the trailer, the armor in the manga was armed with an enormous sword.

Manga
In the manga, the nature of the armor is revealed as a Possession-type Grimm, created by the Schnee Dust Company by amalgamating many captured Grimm specimens. It is described as being comparable to a mecha-suit in strength.

When Weiss reveals her intention to study at Beacon Academy rather than Atlas, the President's Secretary unleashes the Grimm-possessed suit of armor as part of a "test" of Weiss' worth, set up by her father. When the armor attacks, Weiss fights back with Myrtenaster.

In the subsequent fight, the armor lands a hit on Weiss' eye, scarring her, but she ultimately manages to defeat her opponent.

RWBY


In the "White" Trailer, Weiss fights the armor in the White Castle. The exact context of the fight is not given, and it is thought to be either a flashback or a vision. Although Weiss is successful in landing a multitude of blows on the armor, they do not appear to have much effect on it. In retaliation, the armor lands a serious hit on Weiss, causing her to bleed from her head. However, undeterred, Weiss ultimately manages to defeat the armor using a combination of her Glyphs and Dust abilities with Myrtenaster.

Much later, in "Lessons Learned", Weiss is tutored in her summoning by her sister, Winter Schnee during the Vytal Festival. Weiss, who has always had trouble mastering summoning, is apparently unable to summon anything during her training session, much to her frustration and disappointment. However, unbeknownst to her, it is revealed that she was able to manifest a small version of the armor's sword.

In "Heroes and Monsters" after Weiss rushes to Velvet's aid, she manages to summon the sword and an arm in their proper sizes, which results in the destruction of an Atlesian Paladin-290.

In "Two Steps Forward, Two Steps Back", after much practice, Weiss is finally able to fully summon the armor, causing damage to her room in the Schnee family estate in the process. Despite being a full summon, it is smaller than the sword and arm Weiss summons prior.

Abilities and Powers
The Armor appears to have a high degree of swordsmanship and is rather swift for its size, being able to leap high above Weiss' head for a downward slash with minimal effort.

The armor appears to prefer using the momentum of its long sword in motion to execute wide swings, and often chains together blows. It is also quite durable, being slashed at multiple times by Weiss with no visible damage taken, as well as being resistant to various Dust techniques.

It can also seemingly wield its longsword one-handed without any difficulty, while still maintaining the force of a two-handed blow, demonstrating a massive amount of physical strength.

Its unarmed attacks are very powerful, as they have been shown to make contact with and injure Weiss. However, they are slow, predictable, and were shown to be the Armor's downfall, as it wasn't able to strike Weiss through her series of rolls and dodges. This suggests that the Armor was trained primarily in swordsmanship, rather than unarmed attacks. The true nature of the Armor is unknown.

Trivia

 * Some pieces used in Remnant: The Game appear to resemble the Armored Knight, as seen in the episode "Welcome to Beacon".
 * In "Breach", Weiss uses ice Dust to create an ice version of the Armor Knight's sword.