My apologies for the wall of text.
TL;DR version, Everybody hated/looked down on everybody at one point, but relations are getting better.
Side related to the Ainu (descendants of the original aboriginal inhabitants of Japan) Historically, the Japanese don’t consider Okinawans, Japanese (naichi, those from mainland Japan). This geographically and historically correct since Okinawa has its own unique culture, language and its own king before annexation by Japan.
I have a pet theory (supported by the fact that Japan was ready to admit defeat in WWII to prevent the invasion of mainland Japan) that the invasion of Okinawa was acceptable since it Japanese by annexation only. There are reports that during the Battle of Okinawa, some Japanese used Okinawans as human shields and some Okinawans were killed to prevent their possible passing on of military secrets through their (incomprehensible to the Japanese) use of their native tongue.
Acceptance and admiration of Okinawan singers grew tremendously in the mid-90’s when Okinawan singers/groups Amuro Namie, MAX and Speed began to hit the charts. Amuro Namie who retired this year, is the top JPop singer in Japanese history.
I’ve befriended older Chinese, Korean and Filipinos who have told me outright, “You know, we couldn’t have been friends before.” And “I never thought I’d be such good friends with a Japanese [person]”, referring to pre and WWII events.
I’ve had Chinese and Korean video store owners be impressed by interest in their movies. When I explain that I’m Japanese/Okinawa, they laughed and said “Ah, close enough! You’re Korean!, “You’re Chinese!”. Conversely, I’m largely snubbed and looked down upon when I visited Japanese video stores because I don’t know how to read or speak Japanese.
In 2016, KPop group Twice debuted with 9 members, 3 Japanese and 1 Taiwanese, an unprecedented mix. They’re currently the top girl KPop group in Korea and are making huge inroads into Japan (which has been largely resistant to KPop) primarily because of their Japanese members.
Following the success of Twice and their inroads into Japan, this year, another KPop group IZOne was formed through popular voting on a reality elimination show (Produce 48) with 24 Korean and 24 Japanese contestants vying for a place in the final 12 member group. There was controversy because the voting was limited to Koreans only and rumors of limits on how many Japanese would be allowed in the group (it’s comprised of 9 Korean and 3 Japanese).
*Justification was because the winners (I.O.I) of the 2015 version of the show (Produce 101) were highly influenced by International voting. I.O.I (comprised of 10 Koreans (one half Dutch and one Mainland Chinese member) was supposed to represent the South Korean popular choice. This was before Twice debuted and showed that a mixed nationality KPop group could be highly successful.
Also this year, the Chinese version of Produce 101 was aired (with Chinese contestants only) and Rocket Girls was formed. Like Produce 48, voting was limited to China only. And there was some controversy because two of the contestants (who eventually place 1st and 2nd in Rocket Girls) were/are part of a KPop group. Early on there some complaints that they shouldn’t be allowed to participate because of their membership in a KPop group. Some even saying they were no longer Chinese enough because of the time spent in Korea.