Biff the Understudy was a human male fighter and an actor in Raelis Shai's theater performance at the Five Flagons Inn in Athkatla in 1369 DR. He served as an understudy for Haer'Dalis, who was absent. Biff messed up his lines, prompting booing and causing the play to be abandoned.
"Biff the Understudy" is a meta-character in the Baldur's Gate computer game. Many conversations in Baldur's Gate require a certain NPC, and in case such an NPC has been accidentally killed or otherwise not present, the game summons up Biff the Understudy to say the required lines. Biff can actually join the party in some instances, though his statistics leave something to be desired.
He also has a brief cameo as a minor character in Baldur's Gate II: Shadows of Amn, as a genuine understudy in Athkatla. However, he is not necessarily present as a meta-character in Baldur's Gate II owing to a new feature, in which characters who need to flee or speak lines before dying are in fact impossible to bring below 1 hitpoint. Some other plot-essential characters will also, upon becoming hostile or being killed, simply cause an invulnerable enemy to appear who will instantly slay the protagonist and all party members to prevent the game from proceeding in an unwinnable state.