Make the teriyaki sauce. In a small saucepan, whisk soy sauce, pineapple juice, brown sugar, rice vinegar, sesame oil, garlic, ginger, and sriracha if using. Bring to a simmer over medium heat.
Stir in the cornstarch slurry and simmer 1–2 minutes until glossy and slightly thickened. Remove from heat and let cool.
Marinate the chicken. Place chicken pieces in a bowl or zip-top bag. Add about half of the cooled teriyaki sauce and toss to coat.
Reserve the rest for glazing. Marinate in the fridge for 30 minutes and up to 4 hours.
Prep the grill. Heat a gas or charcoal grill to medium-high (about 400–450°F). Clean and oil the grates to prevent sticking.
If using a grill pan, preheat over medium-high and oil lightly.
Build the skewers. Thread chicken, pineapple, bell pepper, and red onion in a repeating pattern. Do not pack too tightly—leave a little space so heat can circulate. Aim for 5–6 pieces of chicken per skewer.
Grill the skewers. Place skewers on the grill and cook 10–12 minutes total, turning every 2–3 minutes.
Brush with some of the reserved teriyaki during the last few minutes. The chicken is done when lightly charred and it reaches an internal temperature of 165°F.
Glaze and rest. Transfer skewers to a platter and brush with more teriyaki. Let rest 3 minutes so juices settle.
Finish and serve. Sprinkle with sesame seeds and green onions.
Add a squeeze of lime if you like. Serve hot over rice or with a crisp salad.