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 the bowl 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, allowing excess marinade to drip off. Reserve the marinade for basting.
Place the chicken on the grill, skin side down, and cook for about 6-7 minutes. Flip the chicken and baste with the reserved marinade. Grill 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.
Garnish with sliced green onions and sesame seeds before serving.