In a large bowl, whisk together soy sauce, ketchup, brown sugar, pineapple juice, rice vinegar, vegetable 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. If using, lightly oil the grill grates to prevent sticking.
Remove the 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.
If using pineapple slices, place them on the grill during the last few minutes of cooking, turning until grill marks appear.
Remove the chicken from the grill and let it rest for 5 minutes. Garnish with chopped green onions before serving.