All Heroes is the menu within the Armory where you can Compare, Retire, Upgrade/Inspect, Level Up, Evolve, Favorite, and Upgrade Rarity for your Heroes. You can sort them by All, New, Level Up (will not be listed if you do not have enough XP), Soldiers, Constructors, Ninjas, and Outlanders. You can view them by Perks, Stats, or Support / Tactical Squad Bonuses.
You can compare Hero stats side by side for two heroes at a time. This will show Health, Health Regen Rate, Shield, Shield Regen Rate, Shield Regen Delay, Hero Ability Damage, Hero Healing Modifier, Run Speed, and Sprint Speed. You can also compare the Perks each Hero has.
Retiring heroes returns most of the XP put into them, or a small amount if they were not given XP. Heroes of Rare, Epic, Legendary, and Mythic rarity also give Training Manuals. Uncommon Heroes do not give any Training Manuals.
You can view your Hero's Rarity, Class/Subclass, Power Rating, Evolution Rating, Level, Stats, Perks, and Support or Tactical Squad Bonuses. 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 Hero, raising the stats drastically and unlocking new abilities at 2 and 3 star Evolution Ratings. Lastly you can upgrade the Rarity of your Hero (for example, from Rare to Epic). This requires Flux and Training Manuals. Upgrading the Rarity of you Hero will not reset them to Level 1, and they will retain their current level.
Normally, Heroes that are in your All Heroes tab will be suggested in the Collection Book (it will make a number appear by the tab associated with the Hero's Class). By selecting Upgrade/Inspect, you can toggle Favorite. This will put this Hero in front of all others (sorted by Power Rating) and hide them in the Collection Book. Even if you select that specific Hero in the Collection Book, it will say "No items available" so you can never accidently slot your Hero.