The following is based on Fallout cut content and has not been confirmed by canon sources.
Rae is a confused super mutant in 2161. He is a freshly dipped pure strain human who experiences feelings of sorrow and loneliness for an unclear reason... until the Vault Dweller meets him, who has the ability to clear them up and recruit him against the Master.
There was at some point a cinematic planned when getting Rae to kill the Master as detailed in PIPBOY.MSG, line 87: (under the section/heading "Movie archive titles"). There is no such cinematic in any of the Fallout 1 data files, however.