Upon its defeat, the player is rewarded the Steel Sword.
The Steel Guardian is a large, muscular creature created from steel and the player-character's own desires. It is a bipedal being that is primarily bluish-silver in coloration with large wings sprouting from its back, blade-like growths protruding from the parent extension, and chains also appear to sprout from the wing's steel "bone" and hang down with the sharp spikes. Its wings attach to its back by a thick, hollow circle infused into his shoulder-blades, and also from this object, sharp-edged spikes grow around its entire circumference. Its legs are well-protected by a set of armor: a greave, poleyn, and cuisse. Its lower arms also appear to be guarded by a vambrace. The Steel Guardian has three toes (one which faces backwards) and four digits (one acting as a thumb) on each hand, all ending with treacherous, sickled claws. The boss's mouth resembles a bird of prey's beak.
The Steel Guardian represents the "Steel" element, as well as the protagonist. It is described as a sadistic, powerful creature birthed of steel and the protagonist's own desires. This creature exists as an avatar by which the protagonist alters reality.
- The Steel Guardian is one of the few monsters that are conscious and can speak in their monstrous forms, the others being Mary, Prisoners, Twin Victim, Bottom, Leonard Wolf, Momma, Bloodied Bag, Monocle Man, Alessa, Lucas, Ben, and Karen. Note that the Sad Daddy could speak only in its human manifestation.
- Its appearance and design is noticeably very angelic, suggesting he may be an angel in the same manner as Valtiel.