Answer: Pepper
Stark manages to knock Killian off him with a well-timed attack, severing the scientist's hand before it melts through the floor. The subsequent damage leads Pepper to fall through the floor and be brought out along the tanker by the moving machinery. Tony struggles to reach her but the tanker jolts and she falls hundreds of feet to the ground, just out of his reach. Enraged, Tony turns around to find Killian still alive and the battle recommences between the two of them. As every hit against Stark takes off another piece of his Iron Man outfit, the fight continues to rage on, proceeding through several iterations of the suit until his most recent prototype returns, placing itself on Killian's body and self-destructing. Tony crashes to the ground without a suit and, unfortunately, Killian survives the blast, reemerging from the flames and rebuilding. He stalks after Tony, but he's saved at the last moment by Pepper who, having survived the fall due to the Extremis testing, manages to acquire one of the Iron Man suit arms and use it to blast Killian to kingdom come.
Pepper, worried that she could be killed by Extremis, is assured that Tony will find a way to work out a cure. To demonstrate his love for her, he also ends up blowing up all of his old Iron Man models, finally letting go of his anxiety.
Trevor Slattery is arrested by the police, as is the Vice President. Pepper survives Extemis-free and then Tony goes under the knife himself to have the shrapnel removed from his chest. In Tennessee, Harley is treated to a lab all his own for how he helped Stark out.
In the end, Tony takes his old ARC chest-piece and tosses it into the ocean. He claims to be a changed man as he drives off with some of his old tech. Despite everything that happened and everything he gave up, he's still Iron Man.