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Characteristics
The Mystery Guardian is smaller than even the Guardian Scout, with a distinctive egg-like appearance and a predominantly white color scheme; it appears to be able to communicate through chirps and whistles, but it is unclear what it is attempting to say.

It does not seem to be able to fight on its own, but it has been shown to be capable of reactivating ancient technology and can travel through time.

Biography
The Mystery Guardian is initially seen in a room in Hyrule Castle emerging from a box, at roughly the same time that Zelda's powers awakened. As it comes under attack by a Guardian Stalker, it escapes by opening a portal that takes it back in time to before the Great Calamity began, presumably in an attempt to change the course of history.

Link and Impa discover it in a dormant state during the Battle of Hyrule Field, where it is reactivated by Impa's Sheikah Slate and consequently saves them by making a Sheikah Tower rise out of the ground. The Mystery Guardian then runs off to find Zelda, who notices that the Slate reacts to the Mystery Guardian's presence.

Link, Zelda, and Impa take the Mystery Guardian to Robbie and Purah, who confirm it to be a previously unknown type of Guardian. According to them, it acts as a power amplifier for ancient technology. Additionally, they are able to extract part of its memory, revealing the devastation that will occur if the Calamity is not stopped.