Dennel was a human who was born in 2481. In 2494, when he was thirteen, he was imprisoned on Justice Beta for arson. He started a fire as a distraction while his father robbed a bank. Dennel was sentenced to seventeen years in prison (he referred to this as a "minor charge") while his father went to a separate prison on Justice Epsilon.
While in Detention Centre 6 on Justice Beta, Dennel became a 'block-walker' - an individual prisoner who patrolled at night, looking for unrest, and was afforded some extra privileges by the prison's real wardens for doing so.
After serving seven years of his seventeen-year sentence, he met and befriended Rose Tyler. He helped the Ninth Doctor and her defeat the Blathereen. (PROSE: The Monsters Inside)