The defence called Peet van Zyl to the stand in an attempt to bolster Pistorius' account that he mistook Steenkamp for an intruder, providing testimony about a loving relationship and a fear of crime that may have pushed the Paralympian to fire through a closed toilet door.
Van Zyl faced tough questioning from the chief prosecutor, however, about Pistorius' alleged egotism and tantrums, high-speed driving and love of guns.
Pistorius, who is free on bail, faces 25 years to life in prison if found guilty of premeditated murder, but he could also be sentenced to significant time behind bars if convicted of murder without premeditation or negligent killing. He also faces gun-related charges.
The agent, who helped guide Pistorius to success as a globally renowned athlete with lucrative sponsorship deals that have since been stripped away, testified that his client had a "heightened sense of awareness" and appeared preoccupied with security at times.
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"He wanted to ensure that we are safe and not being followed," Van Zyl said.
He also remembered a time when Pistorius grabbed him by the arm in apparent fear when the pair heard a loud bang while walking in New York City, and described two occasions in which the runner lost his temper but was not aggressive under "abusive questioning" from journalists.
Prosecutor Gerrie Nel tried to pick holes in Van Zyl's testimony, pressing the agent for details about a reported incident in which a South African athlete who was sharing a room with Pistorius asked for them to be separated because Pistorius was allegedly arguing frequently on his telephone.
He also referred to a 2012 Paralympics race in which Pistorius accused the winner of breaking the rules by using prosthetic limbs that were too long.