Combine first 6 ingredients in a small saucepan, and bring to a boil. Cook 1 minute, stirring until sugar dissolves. Cool slightly.
Thread 1 onion piece onto a 10-inch skewer, 1/2 inch from one end of onion. Thread 1 tofu cube onto skewer. Closely wrap onion piece around and over tofu cube; thread other end of onion onto skewer, 1/2 inch from end of onion. Repeat procedure 3 times to form 1 kabob with 4 tofu cubes and 4 onion slices. Repeat procedure with remaining skewers, onion, and tofu.
Brush kabobs with soy sauce mixture; coat kabobs with cooking spray. Place kabobs on grill rack coated with cooking spray. Grill 3 minutes on each side or until browned, basting frequently with the soy sauce mixture.