so im a musketeer and i got sniper after that i beated up some wharf rats but i noticed a lot of my companions have "pirate" over their heads whenever they got near a bad guy and had a bonus attack even when i didnt chose for he or her to attack that guy.
"Pirate" is a buff some companions have (and that your Pirate can train) that gives you extra attack power when you have boarded another ship. I know Ratbeard has it, for sure. Don't know of any other companions specifically.
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I dont know if all my companions get it but I know a few do. Ratbeard, Sarah Steele, Lucky Jack Russel and Subodai are the ones I've seen get the buff. I have it on my Swashbuckler as well and its a nice little boost when you jump to the enemy ship.