In a large bowl, whisk together soy sauce, brown sugar, ketchup, pineapple juice, rice vinegar, sesame oil, garlic, ginger, black pepper, and red pepper flakes until well combined.
Add the chicken thighs to the marinade, ensuring they are fully coated. Cover and refrigerate for at least 1 hour, or overnight for best flavor.
Preheat your grill to medium-high heat. Remove chicken from the marinade, reserving the marinade for basting.
Place the chicken on the grill, skin side down, and cook for about 6-8 minutes. Flip the chicken and baste with the reserved marinade.
Continue grilling for another 6-8 minutes, or until the chicken reaches an internal temperature of 165°F.
Remove the chicken from the grill and let it rest for 5 minutes.
Garnish with sliced green onions and sesame seeds before serving.