Combine sugar, soy sauce, water, onion, garlic, pepper, and sesame oil in a blender; process until well blended.
Put chicken into a large glass or ceramic bowl. Pour marinade over top and toss to evenly coat. Cover the bowl with plastic wrap and marinate in the refrigerator for 12 to 24 hours.
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Remove chicken from the marinade and shake off excess. Discard the remaining marinade.
Cook chicken on the preheated grill until slightly crisp on the outside, no longer pink in the center, and the juices run clear, about 4 to 8 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).