One of Desmond's dog guards, Geri was found lame and abused by his raider masters. The situation was soon rectified after Desmond tracked the raiders down, trussed them up, and fed them alive to mole rats. Geri has been with him for years and has battled with loyalty and obedience, traits Desmond demands.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
At some point during development it was intended that Desmond would offer Geri to you as a companion, similar to Dogmeat. It is unclear how far this idea progressed during development, but certain resources still exist in the add-on to go with this intended feature. In particular, certain script variables still refer to this option (such as the variable "DogFriend") and Geri is set to use some (but not all) of Dogmeat's dialogue options pertaining to "adopting" him, as well as an AI package titled DLC04DesmondDogPlayerPackage, though the package is only set to ensure Geri stays in the mansion.
End of information based on Fallout 3 cut content.