Shroom, or Skulls, is a User. He's an enigma even by User standards. He has many forms, but favours that of an android armed with quad RPGs.
When Blackout attacked BZPower and destroyed the prison which held many banned users, Shroom helped round up the escapees, including User John Sheppard.
He appeared again on a desert planet, and met Shroom, Moss, and Yuuka in an empty building. His purpose in doing so is unknown, because all he did was give them a long list of his titles.
In the far future, he will fight Ynot and try to stop him from wiping out the Users, but will be killed by his own grenades.