BBOL: Detailing the Defender Role

First one off the ranks is the Defender Role. 

This role's main focus is to protect their comrades and to step in harms way if need be.  This is highlighted by the Sentinel Feat.  Here is the feat description for you

"Once per round, after you have expended your next major action to perform a defend action, you get an additional major action that must be spent before the start of your next turn to make another defend action. Multiple tiers of this feat grant additional major actions usable only for defend actions."

You can only use the Defend Action as an interrupt on another person's turn to increase their defense against an attack, either by using a shield, pushing them out of harm's way, or shouting words of encouragement.  Normally you can only do this once before you're next turn but thanks to Sentinel Feat, you can do it twice. Also thanks to the defensive properties of your equipped shield and the Defensive Mastery feat, you can add +2 to the defend action.

Now for the offensive side of things, by using Bolster on yourself or allies, and Provoke. You can buff others and attempt to force enemies to attack you. Though be aware that when you attack an enemy, you can inflict the Provoke Bane on another.