Whisk together ketchup, brown sugar, soy sauce, pineapple juice, olive oil, rice vinegar, garlic, ginger, sesame oil, salt, and pepper.
Place chicken in a resealable bag, pour in the marinade (reserving ½ cup), seal, and refrigerate for at least 1 hour.
While marinating, soak skewers in water. Toss onion, bell pepper, and pineapple with olive oil, salt, and pepper.
Thread chicken, onion, pepper, and pineapple onto soaked skewers, alternating ingredients.
Preheat grill to 400°F, brush grates with oil, and grill skewers for 5 minutes. Brush with reserved marinade, flip, and repeat.
Grill 4 more minutes until chicken is fully cooked (165°F inside). Remove from grill and serve warm.