In a large bowl, combine soy sauce, banana ketchup, brown sugar, white vinegar, minced garlic, chopped onion, ground black pepper, paprika, and water. Whisk until well blended.

Add the boneless chicken thighs to the marinade, ensuring each piece is coated. Cover with plastic wrap or transfer to a resealable bag and refrigerate for at least 2 hours, preferably overnight.

About 30 minutes before grilling, remove the chicken from the fridge and preheat the grill to medium-high heat (about 400°F or 200°C). Soak bamboo skewers in water.

Thread the marinated chicken onto soaked skewers, leaving space between pieces for even cooking.

Place skewers on the grill and cook for about 5-7 minutes per side, basting with leftover marinade while grilling.

After about 10-14 minutes of grilling, check the internal temperature to ensure it reaches 165°F (74°C).

Remove skewers from the grill, let them rest for a few minutes, and serve hot.