Agathion

Agathion is a being referenced by Index to refer to Cleaning robots. It has yet to appear in the Toaru Majutsu no Index universe.

Principles
Not much is known about the Agathions, but seeing as the kanji used for it translates as Familiar, one can presume that Agathions are supernatural entities believed to assist witches in their practice of magic. Agathion is also, specifically, a name for a familiar spirit that appears only at mid-day. It takes the shape of a man or a beast, or even enclose itself in a talisman, bottle, or magic ring.

Index Arc
Index first references the cleaning robots in Touma's apartment building, and is amazed by Academy City's level of technology for creating "mechanized" Agathions. She later chases them, thinking that the things took her headdress.

Poltergeist Arc
Index reappears in the Toaru Kagaku no Railgun anime adaptation, chasing a Cleaning Robot, still referring to it as an Agathion. She is stopped by Tessou Tsuzuri, and Index tells her that the Cleaning Robot took away the doughnut that she dropped on the ground. She later gives chase to it again after it moves away.