Heat water in a pot until boiling. Cook eggs for 7-8 minutes. Take immediately from hot water and keep in cool water. Peel shells.
Cook soy sauce, sake, mirin, and sugar in a small pot, and boil for a minute. Let cool.
Put eggs and the sauce in a plastic bag with zipper and marinate in the fridge for a few hours to overnight.
Remove the eggs from the sauce (save the sauce). In a bowl of warm steamed rice, add 1/4 cup of the sauce and mix.
Wet hands with water and place 1/4 rice in one hand, and press and form into a triangle shape onigiri. Push down the center and make a well. Place the egg and cover with the rice. Press and re-shape into a triangle.
Wrap with a seat of Nori seaweed.