Mix soy sauce, water, brown sugar, rice wine vinegar, mirin, sake, garlic, and pepper in a bowl to make a sauce.
Place chicken in an electric pressure cooker (such as Instant Pot(R)). Pour sauce over chicken. Close and lock the lid. Select Meat function; set timer for 12 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove lid. An instant-read thermometer inserted into the center of the chicken should read at least 165 degrees F (74 degrees C); if not, cook chicken for another 2 to 4 minutes.
Remove chicken from the pot and cut up or shred. Mix with the sauce from the pot.