From the Ashes

Tengen Uzui's youngest brothers had died from their father's intense training. His brother's name was Kanade Uzui. He was 7 when Tengen, who was 15, killed him. When Tengen was 20 and had visited Kadade's grave, something had happened that caused Kanade to crawl out of his own grave.
Tengen and Kanade had changed Kanade's name and surname, as it would be suspicious since the Hashiras know that Tengen's siblings are all dead except that he was two years younger than Tengen. They changed Kanade's name to Phoenix Uzuya (uzuya similar to uzui). Phoenix is no longer human, but isn't a demon either; he is now a hybrid. While Phoenix still needs blood, with small control from Tengen, Phoenix has created his own breathing technique called Shadow Breathing, a twist on Sound Breathing.
Beyond Tengen and his wives, only Naomi Takayama-Hinatsuru's younger sister, a Hashira of Eclipse Breathing, and Phoenix's childhood best friend-knows about Phoenix