In a large mixing bowl, combine soy sauce, banana ketchup, brown sugar, white vinegar, minced garlic, chopped onion, ground black pepper, salt, and paprika. Whisk until the brown sugar dissolves completely.

Add the boneless chicken thighs to the marinade, ensuring each piece is well-coated. Cover the bowl and refrigerate for at least 2 hours, ideally overnight.

Preheat your grill or grill pan to medium-high heat, around 375°F to 400°F, about 30 minutes prior to grilling.

Thread the marinated chicken thighs onto soaked bamboo skewers, leaving about ½ inch of space between each piece.

Place the skewers on the preheated grill and cook for 5-7 minutes on each side, basting with any remaining marinade.

Once cooked, remove the skewers from the grill and let them rest for about 5 minutes before serving.