I get where you coming from, but Sonic isn't really a good example of that. Sonic is definitely huge worldwide and Eggman or Shadow would be easily recognizable by Japanese.
I wanted to comment on this
You'd be surprised. While I'm sure Sonic is known of in Japan, the Difference in Popularity between Japan and other regions of the World is actually a very large gap, and has been a thing since even the First Sonic Game
Whatever sales a Sonic game does in the West for Example, You can probably 1/8, or even more of that of the sales, which would be the Sales in Japan
As an example, Sonic Colors, the game that was said to have brought Sonic back to the Limelight and was a Classic on the Wii, only did like 10,000 Sales there, which is not the best.
As an early example, Sonic 2, which sold millions in other Regions, only sold 160,000. I had actually thought the Classic Era games were the most successful ones in Japan, but it is actually SA1 and SA2, which had around 500,000 and 400,000 sold respectfully. That's really good for Sonic, but that's actually the height of the Series in sales, and that doesn't scream Block Buster Iconic hit that the West sales will tell of
As an example, that's around the level of Banjo and Kazooie's sales, which was around 400,000, and people considered Banjo more of a Niche character in the West. It's also less than Crash, which had a Million seller, unlike Sonic.
Sonic Adventure 2 in Particular is a well regarded game there, but since it's about 2 Decades old, and it hasn't be released on Nintendo platforms since it's initial re-release on Gamecube, newer gamers might not be as familiar
It's also worth nothing that the 3rd Season of Sonic X was due to wanting to give more for the West, not Japan. I'm sure it aired there, but the reason Season 3 exists is because of the Western side of things
EDIT: Season 3 actually never made it to Japan apparently, which just proves the point even further
This is something I've taken note of when I've speculated on Shadow. I know that i've debated against the other arguments like Shadow being an Assist, "He could only be an Echo", and other Sonic characters, but I think the main thing I can't debate is that Sonic as a franchise is very niche in Japan, especially when you compare it to the Pedigree of it's status in the West. I believe I have mentioned it in the past that this could be a Deciding factor in Nintendo being apprehensive of Choosing Another Sonic character, but If I didn't, I have done so now
Now Sonic is no Rayman or Spyro in Nichehood, and it's especially not no Mortal Kombat, as even though Sonic is more of a Niche in Japan, Sega still of course shows support for the Japanese audience there. But I think the advent of the Tweet implying a More Western focused character does fit Sonic to a T, despite being owned by a Japanese Company