Pistorius involved his girlfriend in conversations about business and wanted her to travel with him so she could see what his life was like, Peet van Zyl said on the stand at the athlete's murder trial.
He was excited about taking her to a concert by Italian opera singer Andrea Bocelli, the agent said.
The double amputee sprinter is on trial for killing Steenkamp in February last year. He admits killing her but says he mistook her for a burglar and thought he was defending himself.
The prosecution says the two had an argument and he killed her intentionally.
Van Zyl described the sprinter as "hypervigilant" and "always fidgety," saying Pistorius once grabbed his arm when they heard a bang on a street in New York, and liked to sit where he could see the exits when they went to restaurants.
But he denied that the athlete was prone to outbursts of anger.
Van Zyl had lost his temper more often than Pistorius during the years they worked together, he said.
The main Pistorius defense lawyer, Barry Roux, questioned van Zyl Tuesday morning before handing him over to prosecutor Gerrie Nel.
Nel, an aggressive, sometimes sneering interrogator, attacked van Zyl from the outset, pressing the agent to say part of his job was to deal with Pistorius' bad press.
But Nel did not question van Zyl for long before requesting an adjournment for the rest of the day. He said he was not expecting van Zyl as a witness and needed more time to prepare to question him about the athlete's character.
The hearing concluded for the day before lunch and will resume on Wednesday for its 36th day.
Screams like a woman?
Van Zyl took the stand after an acoustics expert, Ivan Lin, was cross-examined about sounds on the night of killing.
Some neighbors have described hearing a woman's screams between shots the night Steenkamp died, which the prosecution suggests was the argument that led to her death.
But the defense has argued that Pistorius sounds "like a woman screaming" when he's anxious, and says it was his screams that ear-witnesses heard.
On the stand for the defense Monday, Lin said "typically" one can differentiate between male and female screams, but not without exception.
Nel pressed him on the point but made little headway on Tuesday.
The prosecutor did get Lin to concede that state witnesses could have heard screaming from the Pistorius house on the night of the killing from their homes up to 177 meters (580 feet) away.
The trial restarted Monday after a monthlong break when Pistorius was evaluated by mental health experts at the prosecution's request.
They found he was not mentally incapacitated when he shot and killed Steenkamp.
According to the findings by an independent panel of doctors, Pistorius did not suffer from a mental defect or mental illness at the "time of the commission of the offense that would have rendered him criminally not responsible of the offenses charged."
The report added that "Mr. Pistorius was capable of appreciating the wrongfulness of his act."
Had the doctors deemed Pistorius mentally incapacitated during the shooting, the trial would have immediately ended in a verdict of not guilty by reason of mental illness.
At the trial's conclusion, Judge Thokozile Masipa will have to decide whether Pistorius genuinely made a mistake or killed Steenkamp intentionally.
If she does not believe the athlete thought there was an intruder, she will find him guilty of murder and sentence him to at least 15 years in prison and possibly life. South Africa does not have the death penalty.
If Masipa accepts that Pistorius did not know Steenkamp was the person he was shooting at, she could find him guilty of culpable homicide, a lesser charge than murder, or acquit him, according to CNN legal analyst Kelly Phelps.
A verdict of culpable homicide would leave the sentence at Masipa's discretion.
CNN's Brent Swails contributed to this report from Pretoria.