In a large bowl, whisk together soy sauce, ketchup, brown sugar, pineapple juice, rice vinegar, vegetable oil, garlic, ginger, sesame oil, 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 up to overnight for more flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade, reserving the marinade for basting.
Place the chicken thighs skin-side down on the grill. Cook for about 6-7 minutes, then flip and baste with the reserved marinade.
Continue grilling for another 6-7 minutes or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5 minutes before serving.
Garnish with sliced green onions and sesame seeds before serving.