In a large bowl, using a hand whisk, mix together the shoyu sauce, brown sugar, onion, garlic, ginger, pepper, oregano, red pepper flakes (if using), cayenne pepper (if using), and paprika (if using).
Add the chicken pieces to the soy sauce mixture and cover the bowl with plastic wrap. Refrigerate at least 1 hour.
Preheat a grill to medium heat. Remove the chicken from the marinade and place on the hot grill. Discard marinade.
Grill chicken for 15 minutes on each side, until an instant-read thermometer registers 165 degrees F.
Remove chicken from grill and let rest on serving platter for 10 minutes before serving.