Simmer the soy sauce, sugar, mirin and garlic in a pan until decreased by half.
Let it cool for a while while you work on the chicken.
Cut the chicken into bite-sized pieces.
In Japan they keep the skin on but remove it if you prefer.
Thread the chicken on to the skewers, alternating them with green onion if you like.
Grill the chicken skewers on both sides without the sauce.
When the meat begins to change color, brush the sauce on to both sides and keep grilling, brushing on the sauce about 3 times in total, and turning until the chicken is completely cooked through.