Leng was a large Cardassian man who survived the genocidal attack of his planet by the Dominion in 2375.
By September 2385, Leng had fallen in with Velok Dekreny and ended up working for him as a bodyguard amongst other more disreputable positions. He was responsible for showing Rakhat Blok around Dekreny's operation, unaware that Blok was really an undercover agent working for Ambassador Elim Garak. (The Fall novel: The Crimson Shadow)