Trim any large pieces of fat and tough sinew from the chicken thigh, then cut it into 8 even pieces, about 4 cm (1 1/2 inches) across.
Add the soy sauce, sake, ginger, and garlic and massage into the chicken for about a minute. Cover (or seal in a bag) and marinate in the fridge for at least 30 minutes.
Preheat the air fryer to 190C (375F). Put the chicken in a tray and sprinkle over about 2 tbsp potato starch, turning to coat lightly. Add the rest of the starch a little at a time in two or three rounds, turning the pieces each time.
Spray the chicken with oil on all sides until the coating looks damp all over and no dry, white starch is visible.
Arrange in a single layer in the preheated basket with space between the pieces. Air fry at 190C (375F) for about 15 minutes. Check partway through: flip small pieces (or all of them if your basket is small), and spray any spots that still look powdery.
Cook until the coating is golden and crisp and the inside reaches 74C (165F). Rest on a wire rack for a minute or two and serve with shredded cabbage and lemon or Japanese mayo.