Cut chicken thighs into 1-inch pieces and set aside.
In a bowl, whisk together soy sauce, mirin, sake, brown sugar, sesame oil, ginger, and garlic until well combined.
Place the chicken pieces in a large resealable plastic bag or a shallow dish and pour the marinade over the chicken. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to overnight for more flavor.
Preheat a grill or grill pan over medium-high heat.
Thread the marinated chicken pieces onto the soaked bamboo skewers, alternating with pieces of green onion.
Grill the skewers for about 3-4 minutes on each side, or until the chicken is cooked through and has nice grill marks. Brush with any remaining marinade during the last few minutes of cooking for added flavor.
Remove from the grill and let rest for a couple of minutes before serving. Serve hot with additional green onions for garnish.