The Keeper is a tremendously ancient Transformer, tasked with guarding the sleeping Primus for all eternity. How he does this with no apparent weaponry or altmode is a mystery.
He tells a damn good story, though.
Marvel comics continuity
When the Autobot Pretenders Grimlock, Jazz, Bumblebee and the Micromaster Rescue Patrol were accidentally sent to Cybertron's core by Emirate Xaaron, they met The Keeper, and (more surprisingly) Primus Himself!
Though Grimlock was skeptical that he was facing the real Keeper of Primus, his doubt was put aside when the Keeper revealed Primus' origin (and by extension the history of the Transformers and Unicron).