Mirror

Mirror is the combined form of the Mini-cons Rook and Crosswise, the parners of the enigmatic Sideways. Little is known about this tiny combiner's personality or abilities, as he is generally seen only as a passenger for the treacherous bulk's vehicle mode, generally separating when his master transforms so that one of his halves can form Sideways's head.

He has no official English-language name, the closest he has being "Twins", as Sideways was originally solicited as "Sideways with Twins" rather than "Sideways with Roook and Crosswise". Thus, one could consider him this decade's equivalent of Bumper.

From Teletraan I: The Transformers Wiki, a Wikia wiki.