Combine soy sauce, ketchup, brown sugar, pineapple juice, rice vinegar, vegetable oil, minced garlic, minced ginger, sesame oil, black pepper, and red pepper flakes in a mixing bowl. Stir until well blended.
Immerse the chicken thighs in the marinade, ensuring they are completely covered. Seal the bowl with plastic wrap and marinate in the refrigerator for at least 1 hour, or overnight.
Preheat the grill to medium-high heat.
Remove the chicken from the marinade, letting excess drip off, and set aside the remaining marinade.
Place the chicken on the grill skin side down and cook for 6-8 minutes.
Turn the chicken over and baste generously with the reserved marinade. Grill for another 6-8 minutes, or until the internal temperature reaches 165°F.
Transfer the chicken to a platter and let it rest for 5 minutes.
Before serving, sprinkle with sliced green onions and sesame seeds.