In a mixing bowl, whisk together shoyu, mirin, sake, sugar, ginger, and minced garlic until well combined. Reserve about ¼ to ½ cup of this mixture for later use as a glaze.

Place the chicken thighs in the bowl with the teriyaki mixture, ensuring each piece is well coated. Cover and refrigerate for at least 2 hours.

Preheat the air fryer to 380°F (193°C) for about 5 minutes.

In a small saucepan, combine the reserved teriyaki sauce with a cornstarch slurry. Stir continuously until thickened, about 3–5 minutes.

Spray the air fryer basket with non-stick spray. Place the marinated chicken thighs in a single layer. Cook for 6 minutes, then flip, brush with glaze, and cook for another 5-6 minutes until the internal temperature reaches 165°F (74°C).

Remove the chicken from the fryer and let it rest for a few minutes before serving.