The razor thin Reedman is the combined form of the Microcons. When looking at him from a certain angle, he almost appears invisible.
Revenge of the Fallen film
Ravage deploys the Microcons to steal the All Spark shard in the base. The Microcons combine into Reedman, who battles the guards. He uses his razor-sharp body to actually slice through a soldier before he promptly returns to Ravage after stealing the shard.
Revenge of the Fallen video game
In the PS2 and WII campaign, the Fallen mentions and tells Starscream to dispatch Reedman.