>>1992238819913884 here, and of course. I'll admit I had a hard time thinking of Cobalion's perk without going straight to the "Making others obey." because that would be exploiting the powers granted to followers.
Cobalion, based on everything I know, is very calm and composed. His perk would pretty much let those traits rub off on the follower, making their resolve much harder to shake. In addition, the perk would allow the follower to become much more able in many areas, an example being in that the user could perform awesome abilities with their weapon of choice because they have the mental dexterity to do so.
tl:dr = Cobalion's perk pretty much makes a follower much harder to discourage. They also gain the mental focus to perform advanced techniques with their weapons. Possibly Blade Mastery if they train.
Picture related as I'm gonna use it as an example. The picture is of Mitsuru Kirijo from Persona 4 Arena performing her "Myriad Arrows" SP Skill Attack, where she slashes an opponent around 13-15 times with incredible speed before finishing off the attack with a powerful kick.
I'm sorry if these are super wordy, I'm just trying my best to explain what I had in mind.