I edited most of the post, and added some pictures and links to videos.
What makes you think that?
He wanted to play the (U) version with Wario.
He knows more about (U) frame data offhand than anyone else I know.
He has the best Kirby in Japan.
His Kirby is better than any other Japanese player's characters, including the best pikas.
Wario and Megu's pika are better than Moyashi and Kikoushi's Kirby.
Moyashi's kirby beat both boom's pika and his kirby.
Samus's relative tier list placement in (J) is about the same as Donkey Kong in (U). Despite this, he consistently wins every tournament.
When Jousuke and Isai did their 12 character battle, guess which character Jousuke used to beat Isai's pika:
I'll give you a hint: It was not his Samus OR his Kirby.
Jousuke did say that if Isai's Pika went up against his Samus, he would lose. I could not find out if they actually played that matchup. From all I could tell, the only time Isai played with pika was during the 12 character battle.
Now, here is my counter argument to your statement:
Isai did not play the best Pika's in Japan, even though he had the opportunity to. ( I imagine Wario would be very upset by this)
Isai showed up to a tournament, that the Japanese specifically changed the date for to coincide with his trip, and did not enter.
To me, it looks like Isai is the one who is scurred.
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From my experience Jousuke is a lot like boomfan. He has been playing forever, albeit out of the spotlight for most of it. He does not practice combos in training mode, instead he only plays online. He is really friendly and has a great sense of humor in person. Unlike some top players in North America, he actually tries to do as much as he can for the smash community in Japan. For this I consider him a legend.