EARTHLOCK

Bonds can be formed once you have three party members. The bond is used to get passive bonuses as the bond increases, as well as charge up the super abilities for your characters during combat. You will select two party members to be grouped together, and any actions those two characters take during battle will charge the super meter. Once you activate the super ability, you'll be able to use special abilities for both of the characters until the charge wears off (each ability uses a different amount, and you get five charges). You can use it all with one character or split it between the two if you'd like.

As for leveling all the bonds, this comes from simply fighting battles together. You'll want to pair one set of characters and fully max out their bond (Level 5), and then switch to a new bond. As with the "Champions of Umbra" achievement, you'll likely have two characters who sat out and did not form bonds throughout the game, so this will take a bit of grinding to unlock later on. The best spot would be grouping 8 enemies at the Tremel Tree. You'll gain one bond per fight from 1-3 stars, then every two fights you'll get from 4-5 stars.