In a large bowl, whisk together the soy sauce, ketchup, brown sugar, 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 2 hours, or overnight for best flavor.
Preheat your grill to medium-high heat. Remove the chicken from the marinade, reserving the marinade for basting.
Place the chicken skin-side down on the grill and cook for about 6-8 minutes, or until the skin is crispy and golden brown.
Flip the chicken and baste with the reserved marinade. Grill for an additional 6-8 minutes, or until the internal temperature reaches 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.