Citadel is an in-game character and member of the Freedom Phalanx.
Originally named "Bastion", he was the first-ever android superhero for Paragon City. He was badly damaged in the First Rikti Invasion of 2002 and was rebuilt using the remains of the power suit once worn by the superhero Horatio.
(Please see his Paragon Wiki entry for more information.)
He also makes a brief appearance in "The Crucible" #12.