Prepare the banana ketchup by mixing all its ingredients in a bowl until smooth.
In a separate bowl, combine banana ketchup, lemon-lime soda, soy sauce, garlic, brown sugar, black pepper, and lemon juice to make the marinade.
Add chicken thighs to the marinade, cover, and refrigerate for 2 to 4 hours or overnight.
Preheat the grill to medium-high heat.
Grill chicken for 7-10 minutes per side, basting occasionally with the reserved basting sauce.
Mix reserved banana ketchup, soy sauce, brown sugar, vegetable oil, and fish sauce to make the basting sauce.
Ensure chicken is cooked through (internal temperature 165°F or 74°C).
Remove from grill and let rest for a few minutes before serving.