The Heroes screen shows all the Heroes you currently have. You can fuse, evolve and sell them here.
The row at the top shows those Heroes in your current squad. Note - you cannot change your squad formation on the Heroes screen - that is done on the 'Team Select' screen before entering a quest.
To view the full details of a Hero, tap and hold on their icon to reveal their Hero Card.
You can use the Filter and Sort drop-down menus to help you to navigate your collection.
Fusing Heroes increases their XP. When their XP is high enough, they will 'level up', making them stronger. The bar on the fuse tab shows how far a Hero is from their next level.
Typically, you would fuse weaker Heroes into more powerful ones. Tap on a Hero to put them in the larger, left hand box on the Fuse tab. Then, select up to five Heroes to fuse into them, and press the FUSE button.
Some Heroes will lock when you are in Fuse mode - this is to protect you from fusing away your best Heroes. If you are sure you want to fuse them away, tap and hold on their icon to bring up there Hero Card, from where you can unlock them and make them fusable.
The best XP boosts come when you fuse Heroes of the same Element, particularly if you are fusing identical Heroes into one another.
Each Hero has a 'max level'. For some Heroes, if you level them up to max level, they can be evolved into a more powerful hero. Clicking the Evolve tab will show you which, if any, Heroes are ready to be evolved. Evolution requires Evo Ingredients, which are generally acquired by playing Vortex Quests.