In a large mixing bowl, combine soy sauce, banana ketchup, brown sugar, white vinegar, minced garlic, chopped onion, ground black pepper, paprika, and water. Stir until the sugar is dissolved.

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

Preheat your grill or grill pan to medium-high heat, taking about 10-15 minutes.

Once marinated, thread the chicken pieces onto soaked bamboo skewers, leaving space between pieces for even cooking.

Grill the chicken skewers for 5-7 minutes on each side, basting with leftover marinade to prevent dryness.

Remove chicken from the grill, let it rest for a few minutes before serving.