Put mirin, soy sauce, sugar and garlic, if using, into a medium-sized sauce pan, and cook over medium heat until there is only half as much sauce remaining.
Let cool slightly while you are preparing the chicken.
Cut chicken into bite-sized pieces; keeping the skin on is usual in Japan.
Thread chicken onto skewers, alternating with green onion if desired.
Begin grilling the chicken skewers on both sides without the sauce, when the meat starts changing color, brush the sauce on both sides, and continue grilling, brushing on sauce about 3 times total, and turning until done.