Bring 2 cups water and wild rice to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until rice is tender, 30 to 45 minutes. Drain excess liquid, fluff rice with a fork, and cook uncovered for 5 minutes.
Bring 1 1/4 cups water and barley to a boil in a saucepan. Cover, reduce heat to low, and simmer until barley is tender, about 30 minutes.
Heat oil in a skillet over medium heat; cook and stir kale until bright green and softened, 3 to 5 minutes.
Mix wild rice, barley, kale, raisins, walnuts, feta cheese, mustard, and black pepper together in a bowl.
