Normally they revive after a battle. There was a bug in a previous version, where this did not happen. That's why, if you're playing an older version, you should update to fix the problem.
You can get the newest version for all systems on the main page at https://ryensaotome.itch.io/renryuu , or if you prefer to download somewhere else you can also get the game at https://mega.nz/#F!CpQXwBDS!wwZlWRrPWoGzZX5h0-MlRQ
The newest version is 20.07.19