What makes Kewpie (Japanese) mayonnaise so different from Hellmann's?
OK, I did it. The rice vinegar was definitely the biggest difference.  A pinch of MSG (Accent) sealed the deal. I found a recipe on Serious Eats. I tasted after every addition, saving the MSG for last. It wouldn't be as close having skipped anything that I used.

I didn't have real hon-dashi or Japanese mustard, but I had some instant miso soup powder and some Colman's mustard. It was close enough. I also used just rice vinegar, I didn't have any malt vinegar.

I don't have any real Kewpie to do a side by side comparison, but I can tell this is pretty close. It's definitely much closer to Kewpie than Hellmann's.

Everything that has been said here was right.