User blog comment:DirgeOfCerberus111/Adeptus Custodes Vs. Grey Knights/@comment-3398633-20111028195740/@comment-3258762-20111028200952

The Custodes were not originally bodyguards, the Companions were. The Custodes were sent as last chance soldiers for legions needing them, as can be seen with Constantin Valdor when he fought with Russ. They became a full bodyguard after the Emperor gell.