In a mixing bowl whisk together ketchup, brown sugar, soy sauce, pineapple juice, 2 Tbsp olive oil, rice vinegar, garlic, ginger and sesame oil. Stir in 3/4 tsp pepper and season with salt if desired.
Preheat a grill over medium heat to 400°F (200°C). Meanwhile, drizzle remaining 2 Tbsp olive oil over red onion, bell pepper and pineapple and toss. Season red onion and bell pepper with salt and pepper, then thread red onion, bell pepper, pineapple and chicken onto skewers until all of the chicken has been used.
Brush grill grates with olive oil then place skewers on grill. Grill 5 minutes then brush along tops with 1/4 cup of remaining marinade. Rotate to opposite side and brush remaining 1/4 cup of marinade on opposite side.