Protector Augusti Nostri (lit 'Protector of Our Augustus') was a title given to individual officers of the Roman army as a mark of their devotion to and approval by the Emperor himself.
It seems to have been granted to officers who had distinguished themselves serving directly under Gallienus in his wars against barbarian invaders of the Balkan and German provinces and Italy and would-be usurpers in those regions such as Ingenuus.
So far as is known it was never bestowed on any officer of senatorial rank – senators were effectively excluded from service in the army soon after the first protectores appeared.
There is contemporary reference to a Princeps Protectorum, but it is likely that this officer's functions were not related to direct command in battle, but were instead ceremonial.
The purpose of the protectores at this time seems to have been two-fold: (i) to encourage a personal loyalty to the Augusti (particularly Gallienus: there is no reason to suppose that Valerian had any real interest in this innovation) among the most energetic and charismatic officers of the Imperial Field Army and thus combat the spirit of military dissent that was tearing the Empire apart at this time; (ii) to promote ambition among the officer cadres of provincial garrisons, which would serve the same purpose.