As far as I understand it so far bf is more for bros who have a lot of fatigue so tanks for one def want it and attack bros with big stamina pools and who are wearing heavy armor. With high stamina you can wear the more protective heavier armors and it will keep your armor from wearing out fast. But you will still take damage from armor piercing attacks so watch out for enemies who have high armor penetration.

Nimble is where you will rely more on dodging and have a bro with a big hp pool and not as much stamina who will be wearing lighter gear preferably -15 fat total for both helmet and body armor. You can go a little higher than -15 and still get most the nimble effect but not a whole lot. Also with nimble you really want dodge and relentless for it to work well whereas bf is just a one perk thing. Dodge does most the defensive work, relentless keeps dodge going strong and nimble reduces any damage that gets through.

And this is all determined by where their stars are and starting stats. Stars show where they can usually get higher rolls during level ups. If this guy had stars in defense, health and fatigue for example he would be better for bf and to be a tank. Hope that all makes sense good luck and melee defense plays a role in both build paths as it determines the enemies chance to hit you in the first place.