The man in the lift, who has been on the other side the entire time and is genuinely pleased when he no longer has to hide it.
Brock Rumlow commands S.H.I.E.L.D.’s counter-terrorism S.T.R.I.K.E. team and is a Hydra double agent. When Rogers refuses to hand information to Secretary Alexander Pierce, Rumlow and his unit are sent after him, and the lift fight that follows is the best action sequence in the film and the moment the series learned what to do with Captain America.
After he is exposed he fights Sharon Carter and Sam Wilson, and is left in the wreckage of the Triskelion.
Minor spoiler
He returns in Civil War burned and armoured as Crossbones, steals a biological weapon in Lagos, and detonates a vest when Rogers has him. Wanda throws the blast upward into a building. The people killed there are the reason the Sokovia Accords exist, and the reason the Avengers break.