According to Amazon, Kewpie mayonnaise contains vegetable oil, egg yolk, vinegar, salt, MSG and spices (no sugar). Hellmann's (which I specify only because it seems like it's the 'canonical' American mayonnaise) contains soybean oil, water, whole eggs & egg yolks, vinegar, salt, sugar, lemon juice, natural flavors and calcium disodium edta.
To me, the Kewpie mayonnaise seems sweeter, even though it doesn't contain sugar, and the Hellmann's does contain sugar. But that's not even the big difference to me. Somehow, the Kewpie is just "right" in sushi rolls and for just about every other application from that part of the word that benefits from mayonnaise. Yet, I find Kewpie just awful on a bologna sandwich. How are they so different?
If I make mayonnaise, the list of ingredients will look more like the Kewpie, but taste more like Hellmann's.