Trim and discard fat from chicken. In a plastic food bag, combine chicken, soy sauce, sugar, garlic, and pepper; seal. Let stand 10 to 15 minutes, turning bag occasionally.
In a 2- to 2 1/2-quart pan, combine rice and 3 1/3 cups water. Bring to a boil over high heat and cook until most of the water is absorbed, 7 to 10 minutes. Cover and simmer over low heat, until rice is tender to bite, 10 to 15 minutes longer.
Set an 11- to 12-inch nonstick frying pan over medium heat. When pan is hot, lift chicken from marinade and place in pan. Cook chicken, turning to brown both sides, until no longer pink in thickest part (cut to test), 6 to 8 minutes total. Lift out chicken and set on a platter; keep warm.
Pour remaining marinade and 1/4 cup water into pan. Stir over high heat until boiling. Pour into a small bowl.
Serve chicken with rice; garnish with onions. Offer marinade to spoon over rice and chicken.