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

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

Preheat your grill or grill pan to medium-high heat, around 375°F (190°C). Ensure the grill grates are clean and lightly greased.

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

Carefully place the skewers on the preheated grill, cooking for about 5-7 minutes on each side, basting with leftover marinade as needed.

Once grilled to perfection, let the chicken rest for 5 minutes before serving.