The influential investor’s widely anticipated live stream started 25 minutes late, with pounding drums and pictures of cats. Suddenly the music gave way to a high-pitched drone as Keith Gill, the stock trader known as “Roaring Kitty,” appeared on camera. Wearing sunglasses and bandages — he never explained why, other than to say they were just for show — he cracked open a beer and spoke to hundreds of thousands of viewers who tuned in for investing advice — and instead got a rambling...