Six-winged and on fire, calling to each other above the throne — and they appear in exactly one chapter.
Seraphim are named as such only in Isaiah's vision. Each has six wings: two covering the face, two covering the feet, two for flying — so four of the six are for not looking and not being looked at. They call holy, holy, holy to one another, and the doorposts shake. One takes a coal from the altar with tongs and touches the prophet's mouth with it. The name means burning ones, and the same Hebrew word is used elsewhere for the fiery serpents in the wilderness, which is why some read them as serpentine.
Disputed. The word means "burning ones" and is used of the fiery serpents of Numbers 21. Whether Isaiah's seraphim are serpentine, and how they relate to the cherubim, is not settled.