You must equip your Characters with Gear in order to increase their overall power level. Tap on a Character's profile in your roster. If Gear is already in your inventory a + symbol will indicate that it is available to be equipped to your Character. Gear that you did not collect yet will be displayed with a "search" symbol. Click on the magnifying glass to inspect the item. Click on FIND to see where to locate it!

When you have equipped all the possible Gear for your character's tier level, you will be given the opportunity to tier up and increase their tier. This will allow you to equip more advanced Gear and continue strengthening your character.