Cohort commander?

[Nathan Ross]

Jim Summerly, actually.

Ha, yes! Sorry about that - the name on the title page is James Robert Summerly, and I wasn't paying enough attention!

The only hint I know for the 1st cohort comes from Vegetius, who says, that the 1st cohort fights on the right wing of the first battleline which is the most important part of an ancient battleline.

Yes, he also mentions (Re Militaris II.6) that the double first cohort contains 'select men as regards birth and instruction in letters' (which might suggest an admin role for some of them?) and protects the eagle, and later (II.9) that these men were 'favoured by their wealth, birth, literacy and strength' (which is an odd combination of attributes, I think!). Nothing else about a bodyguard role.

However, back to our subject, that same section (II.9) contains the note that the cohorts were 'governed by Tribunes or praepositi', who would 'order the soldiers entrusted to them to train daily under their gaze (and) encourage them to imitate their own example, since they were themselves perfect in the art of bearing arms'

This is fourth-century practice, of course, and Vegetius gives the cohort as a tactical unit with its own ensign. Even here, though, the training/admin role is accentuated. But it does still suggest that commanding a cohort (if required) might fall to an officer of tribune level.