The star of Disney's Queen of Katwe died at age 15 in February — over three years after she was diagnosed with a brain tumor. "We mourn the loss of our beautiful Nikita Pearl Waligwa," her costar David Oyelowo wrote on Instagram. "She was a ball of light in @queenofkatwemovie and in life. Her battle with a brain tumor was humbling to witness. Her light will live on."