Andrew Nelson was a man working for Poseidon and a member of 21 Void. He was first seen when Sara approached him to question Jacob's intention for tracking her phone. He was later mentioned by Poseidon as being "smarter than Michael".
Andrew Nelson was then seen later helping the group try to overcome Michael by constructing his tattoos and what they meant, Although he couldn't quite understand what they meant since they were too small to see.
Andrew Nelson was then seen again confronting Sara after she broke into Poseidon's office building. He tried to shoot Sara but was quickly stopped by Lincoln Burrows. Andrew Nelson was then later arrested.