We exist! With the disclaimer that while I am not natively from Japan, I've lived here virtually my entire adult life, over 20 years now, and am naturalized. I am involved with local marxist groups.
Interestingly, Marxism had and continues to have immense influence on intellectual traditions here. It really first came to Japan through universities, and a lot of that continues today. At least at the university and graduate level. A lot of the MEGA project work was done out of Japan, surprisingly enough. If the puppet government and the US hadn't come down hard on post-war communists in Japan (see: Red Purge), I think it is very likely we would be a socialist state today.
There is definitely a lot of resistance towards US influence that has grown in recent years, I won't say it all comes from socialists as much as it comse from people who have just been disenfranchised with the US. This is especially true, unsurprisingly, in regions that are directly hosting military bases, particularly in Okinawa/Ryukyu. Which is basically double occupied by Japan and the US. Theft, sexual assaults, drunk incidents are all pretty common side effects. A helicopter actually lost pieces of the helicopter while flying over an elementary school not that long ago. These are all things that have a lot of people really fed up. All the hot dog and hamburger "Friendship Days" they host can't really make up for these things.
A big issue recently is the relocating of a US base closer to residential areas in Okinawa. Okinawa governor Tamaki refused, so Tokyo courts mandated that he had to approve the permits. The deadline came and went and the governor has refused to do so. While the permits did get rammed through eventually (the governor never caved, Tokyo just went over his head) he delayed everything about four years and significantly eroded public trust in Tokyo.
I don't necessarily think this is a sign of a full decoupling from the US, but it is definitely an element that has significantly impacted the public perception. I think the average person here would still consider the US an ally, but wants to be less reliant on the US economically and militarily.