Temp assets, that are going to be replaced with human-made assets later on. Better than programmer art.
Programmer art makes it immediately recognizable as a placeholder. It's a feature.
I know there was a recent story about some game having AI assets in it that the publisher then claimed were placeholders that they just 'forgot' to replace.

Unicode has over 100,000 kanji, though the vast majority of these are esoteric kanji that are rarely used. You could trim it down to just the Joyo kanji list, consisting of 2,136 characters for everyday use.