As an Asian this whole thread feels so weird and I can't tell if people actually eat rice with ketchup or they are tricking me into trying an abomination
Why can't y'all just make normal children's food like chicken curry with rice? Stop putting so much sugar and corn syrup in everything.
If this continues we'll have to retaliate: see how certain East Asian countries make pizzas and burgers and see how you like it! (PS: it was flatbread with corn and ham as the only toppings)
Oh and the original answer: since so many people have already answered soy sauce, I'd say chicken soup or pork broth.
I wouldn't say it's common, but I also wouldn't say it's unheard of...and I would never put it past Americans to try an odd condiment application.
Honestly though, when you look at the ingredients, it's not too drastically far off from the ingredients of a sauce you might specifically put together as part of a more traditional rice dish: tomatoes, vinegar, onion, garlic, ginger, coriander, cumin... bit heavy on the sugar but a lot of sauces in Asian cooking are even sweeter.
I agree it seems repulsive on the surface to me too, but now that I've been thinking about it...I kinda wanna try it.