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

Add the boneless chicken thighs into the marinade, ensuring each piece is thoroughly coated. Cover the bowl tightly and refrigerate the mixture for at least 2 hours, preferably overnight.

Preheat your grill or grill pan to medium-high heat about 15 minutes before cooking.

Thread the marinated chicken pieces onto soaked bamboo skewers, leaving space between each piece.

Place the chicken skewers on the preheated grill and cook for about 5-7 minutes on each side, or until the internal temperature reaches 165°F (75°C). Baste with leftover marinade while grilling.

Check for doneness by ensuring no pink meat remains inside the chicken. Let the skewers rest for 5 minutes before serving.