Rubella is a jester who can't get rhymes right, you can unlock this achievement after your first flight.
Once you gain the power to fly, fly up and follow the path where you will find a jester crying on a platform. Talk to her and she will join your party. Rubella has healing powers which help ALOT during the game so they should be your first priority when unlocking her upgrades.
Now that you have her, you can fly back to the alter you awoke on and keep going a little past it to fight the giant and his pet dogs.
Note: do not try take the giant on without rubella as he can deal 40+ damage, even with the DLC stardust he will still own you.