The disguised Frumentarius is a spy and messenger. He will appear if the Courier has a positive Legion reputation, to offer an incentive to remain in Caesar's good graces.
After speaking to the player, he will run to Cottonwood Cove and disappear from the map.
If he approaches through hostile areas of the wasteland, he may be killed before he can contact the player and initiate the quests.
Despite being disguised he will still be attacked on sight by enemies of the Legion, such as NCR troopers.
In case he is killed before getting a chance to talk to you, you can enter the following console commands to revive him and initiate the Quest Caesar's Favor/Hire:
prid 00164ED0 -- Selects Disguised Frumentarius
moveto player -- Spawns the dead body next to the player
Now, face the corpse and open the console. Move the cursor over the dead body and left click, then type the following command:
resurrect -- Revives the Disguised Frumentarius
Note: Sometimes the dead body won't spawn, in that case try traveling to a location close to where he got killed.