In a medium bowl, whisk together soy sauce, mirin, sake, brown sugar, sesame oil, ground ginger, and black pepper until the sugar is dissolved.
Add the chicken pieces to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat your grill or grill pan over medium-high heat.
Thread the marinated chicken pieces onto the soaked skewers, alternating with pieces of green onion.
Place the skewers on the grill and cook for about 4-5 minutes on each side, or until the chicken is cooked through and has nice grill marks.
Remove from the grill and let rest for a couple of minutes before serving.