At least some content in this article is derived from information featured in Harry Potter: Hogwarts Mystery. Spoilers will be present within the article.
Suits of armour come in all shapes and sizes, and can be bewitched to move and speak. There are many suits of armour currently Hogwarts School of Witchcraft and Wizardry. During the Second Wizarding War, Professor McGonagall animated several suits of armour to defend Hogwarts against the oncoming Death Eaters. The shields of those suits of armour in the front row are immune to spells, making them ideal for castle protection. During the Second Wizarding War the Death Eaters tried to destroy the armoured by using the Killing Curse, but their shields deflected all oncoming spells. The Death Eaters could not defeat them easily, and they maintained the defensive lines of Hogwarts.
A suit of Armour preparing for battle in the Battle of Hogwarts
Peeves once hid inside a suit of armour over Christmas break during the Triwizard Tournament. As the armours were singing Christmas Carols, Peeves began to fill the gaps between lyrics with crude phrases and words. Argus Filch had to extract Peeves from the suits on occasion.
The suits of armour seem to have a degree of sentience, as a suit's head turned to follow Harry when he was going to Umbridge's Office, and another suit laughed as Neville tripped on the stairs.
While the suits of armour are harmless in the books/films, in some of the video games the Knights also serve as obstacles. In some versions, they attempt to attack Harry with their swords. In the PS2 version of Harry Potter and the Prisoner of Azkaban, the player must sneak past them and avoid their line of vision, else they will stomp and creak loudly when seen.
In Deathly Hallows: Part 2, the shields of the suits of armour in the front row have the House crests and Hogwarts crests on them: the far left a Gryffindor Crest, the second from the left a Hufflepuff Crest, the middle knight the Hogwarts Crest, the second from the right a Ravenclaw Crest, and the far right a Slytherin Crest.
In LEGO Harry Potter: Years 5-7, Suits can be summoned to kill anything, including Giants and, despite what we see in the movies, they are invincible.
