It would have been fun if a "fake" assembly language had to be used (rather than one corresponding to the actual CPU) that MIX from Knuth's books would have been used (yes, current versions of the books use an updated version called MMIX, but plain MIX would have been contemporary for these pocket computers).
The Japanese electronics giants of that era had an alterntive to MIX, in the ITRON  initiative, which included plans for a system-wide virtual machine to be used for compatibility purposes .. Knuths' books, while popular among Japanese computer scientists of the era, had some stiff competition in the Japanese industrial sector .. a few of the major players produced ITRON kernels with this capability, it would be interesting to research which of the personal computing toys of the era were ITRON machines in disguise ..
 - https://en.wikipedia.org/wiki/ITRON_project
I had the Casio fx-780P. It was a piece of junk, even by the standards of the day.
Yet people love them, especially the PC-6. It has to be the form factor.
Meanwhile, my PC-4 (Casio PB-100) was far more useful to me and they seem much more durable.
Beats only having BASIC as my PC-1 was limited to.
This isn't a real assembly language that has direct access to the hardware. Or even the capability to write "Hello world" (the only I/O is numbers, in either hex or decimal).
And labels are limited to 3 characters, so not much better than line numbers in BASIC! The one real advantage would be the 16-bit integer arithmetic, probably the main reason for it being faster than BASIC (which only supports floating point numbers on these machines, implemented in software on a 4-bit microcontroller).
I had one of these too, but without the manual. The "Asmbl" button was fascinating to me, but I couldn't figure out how to use it until much later when I found a scan of the manual online - you can imagine the disappointment! Figured there had to be some educational purpose behind it, but this is the first time I read about this Japanese standard.
I was hoping for katakana opcodes.
You'll love G-BASIC on the Tomy Pyūta then, because its very primitive BASIC is indeed katakana: https://www.floodgap.com/retrobits/tomy/pyuuta/pyuuta1.html
The form factor is not that great. Much later I got a HP95LX then a HP200LX, which were genuinely useful machines.