People tried tracking him back in 2010, turns out some of the emails he sent originate from internet cafe's and public libraries in Tokyo. So the person at least physically resides there. I'd say there is a high likelihood he is of Japanese descent as well.
Why does perfect English have to mean he is not natively Japanese?
Considering his staggering expertise in cryptography, programming and mathematics I wouldn't say it's too far-fetched to think he is extremely well-educated, maybe even self-learned.
Not being from the UK or USA doesn't automatically mean you make grammatical errors, spelling mistakes or use poor language.
#1 jack of diamonds.. so how good's your japanese?
all kidding aside.. never seen a japanese who could speak english fluently and i've worked at the honda plant, in utsunomiya japan, and worked on a toyota program for 5 years. There may be a few but based on personal experience the odds would be 100,000:1 +
To them, speaking english is almost a dishonor...
But some of them can write english passibly.. you'd still notice alot of mistakes/errors unless he had someone edit for him. But 99% likely at least his mouthpiece is not native japanese.
Also i dont think his expertise in cryptography or math is 'staggering' and his programming is definitely not. The dumbest, bucktoothiest native japanese guy can speak japanese better than Stephen Hawking could if he studied japanese for 20 years. IMHO. People overestimate the variation in intelligence on the grand scale, the smartest human is probably 1% smarter than an average human, on a universal scale.