I get why “otherwise unknown Christian” makes you unsure. The writer only tells us his name is John and that he’s a fellow believer going through the same trouble as his readers. He says he ended up on Patmos for holding onto Jesus.
A lot of visionary writings from that era were put under famous names, like Enoch or Ezra. This book doesn’t do that. The author uses his own common name and describes a real exile, with a punishment already handed down. That points to someone the churches could identify, and the authorities as well. I think John wrote Revelation, even if we lost the rest of his story.