All Defenders is the menu within the Armory where you can Compare, Retire, Upgrade/Inspect, Level Up, Evolve, and Favorite your Defenders. You can sort them by All, New, Level Up (will not be listed if you do not have enough XP),Assault Rifle, Melee, Pistol, Shotgun, and sniper. You can view them by Perks or Stats.
You can compare Defender stats side by side for two defenders at a time. This will show Health, Health Regen Rate, Shield, Shield Regen Rate, and Weapon Skill. You can also compare the Defender Perks each Defender has.
Retiring defenders returns most of the XP put into them, or a small amount if they were not given XP. Defenders of Rare, Epic, Legendary, and Mythic rarity also give Training Manuals. Common and Uncommon Defenders do not give any Training Manuals.
You can view your Defender's Rarity, Weapon Class, Power Rating, Evolution Rating, Level, Stats, and Perks. You can view your character's next Evolution Rating stats and requirements as well. Most importantly, you can level up your character using Hero XP up to a multiple of 10 in which you are given the option to Evolve your Defender, raising the stats drastically.
Normally, Defenders that are in your All Defenders tab will be suggested in the Collection Book (it will make a number appear by the Defender tab). By selecting Upgrade/Inspect, you can toggle Favorite. This will put this Defender in front of all others (sorted by Power Rating) and hide them in the Collection Book. Even if you select that specific Defender in the Collection Book, it will say "No items available" so you can never accidently slot your Defender.