If using wooden skewers, soak them in water at least 30 minutes before cooking.
Using a serrated knife, trim the pineapple and cut off the skin. Cut into 1 1/2-inch-thick rounds and then cut into chunks around the core; discard the core.
Cut the onion in half vertically and then separate the onion into 2-to-3-layer portions. Cut the portions into 1 1/2-inch pieces.
Preheat a grill or large grill pan over medium-high heat. Thread the pork, pineapple and red onion onto the skewers. Brush with oil and sprinkle with salt. Grill the skewers until lightly charred on the bottom, about 3 minutes.
Meanwhile, in a small bowl, whisk together the soy sauce with the vinegar, agave nectar and sesame oil.
Flip the skewers and brush with some of the soy mixture. Continue to grill, flipping every 2 minutes and brushing with the soy mixture, until lightly charred all over and the pork is just cooked, 6 to 8 minutes longer. Transfer to a platter, sprinkle with toasted sesame seeds and serve.