EARTHLOCK

There are six total playable characters in the game. The final two added to the party (Taika and Pat) join quite under leveled compared to where the rest of your characters will be when they join, so unless you spend some time getting them up to speed right away, they will likely be left on the sidelines for the majority of the game. You'll have to work on their levels and bonds at some point though.

To level up more quickly, it is beneficial to group together as many enemies as possible. Obviously this will make for a tougher fight, but if you have your super abilities charged, you can easily take on eight enemies at once. You get a multiplier for every extra enemy (capped at 8), so you can rack up a few thousand xp in one battle if you're bold enough.

You'll likely get to Level 20 with your main group before the end of the game, so you'll just need to grind with the other characters a bit.