Cluster was mentioned as a social VR platform in the 2019 infographic published by the San Francisco-based venture capital firm The Venture Reality Fund (which is available here).
Cluster is perhaps best described as a Japanese version of VRChat:

The website is in Japanese only. There’s a video, but it’s in Japanese too:
There appear to be two basic kinds of avatars: the anime kind which you see everywhere in VRChat, and a sort of placeholder robot avatar which reminds me strongly of those in AltspaceVR, but with even less customization options. Here’s a picture of what the latter look like:

I don’t read or speak Japanese, so that’s pretty much all I can say about the product. From the video, it appears there are VR and PC versions of the platform. Last September, the company secured 400 million yen (about US$3.6 million) in venture capital.
Yet another addition to my list of social VR/virtual worlds! Hunting down all these social VR platforms and virtual worlds is kind of like another Japanese game, Pokémon: “gotta catch ’em all!”