Add mayonnaise, soy sauce, mirin, sugar, paprika, garlic powder, and black pepper to a medium bowl.
Whisk all ingredients together until fully combined, smooth, and uniform in color — about 60 seconds.
Add the tablespoon of water and whisk again until incorporated and the sauce is at a drizzlable consistency.
Taste and adjust — add a pinch more sugar for sweetness, a touch more soy sauce for depth, or a splash more water to thin it.
Transfer to an airtight container or jar and refrigerate for at least 30 minutes before serving. Overnight is best.
