Mix soy sauce, calamansi or lemon juice, brown sugar, ketchup, minced garlic, ground black pepper, and salt in a bowl.

Add pork belly to the marinade, cover, and refrigerate for at least 2 hours or overnight.

Soak bamboo skewers in water for 30 minutes if using.

Preheat grill or grill pan to medium-high heat.

Thread pork belly onto skewers or place directly on the grill. Reserve remaining marinade for basting.

Grill for 8-10 minutes on each side, basting with reserved marinade.

Remove from grill and let rest before serving.

Serve with steamed rice and a side of pickled vegetables or cucumber and tomato salad.