Little Billy

Little Billy is a child who constantly encounters extreme misfortune.

TTS Canon
Little Billy wears a cardboard box fashioned like a space marine helmet. He keeps suffering horrible accidents yet somehow survives; because of this, some people speculate that he is a perpetual. This is further supported as he makes an appearance in the BEHEMOTH Trilogy, which supposedly takes place 300 years prior to the events of TTS. To further support this, Perpetuals stop aging past a certain point, which in Little Billys case, is very early.
 * Firstly, Billy and his friends were ran over by Magnus the Red on his Bike.
 * Next, he and his friends were abducted by Dark Eldar and thrown in a slave pit. He watched as the slavers turned his friends into meat puppets.
 * Then, he was the target of Lucius the Eternal's "social experiment," where Lucius psychically contacted him pretending to be a little girl. Confused by the message, Lucius suddenly appeared, and assaulted him with his tentacle appendages.
 * Billy was captured by the Dark Eldar again, but this time was saved by Sly Marbo.
 * Was hammered in the face by Calato.
 * Caught in a time loop by Lucius as part of the opening bit of his Slaaneshmas Special.

Billy may possibly be a mutant, assuming that he is the same Billy that is mentioned by Boy, explaining why he wears a mask at all times.