"Now, I think he's a terribly ill man, and it's difficult to reconcile everything I knew for 40 years and what I know now," the former employee, Daniel Bonventre, said in federal court in New York City.
Madoff, who is serving a 150-year prison sentence, squandered nearly $20 billion from thousands of investors over several decades.
Bonventre has pleaded not guilty to fraud charges. Authorities say he cooked the books to throw off regulators.
He's on trial with four other former Madoff employees.
The defense says Bonventre worked for the legitimate broker-dealer side of Madoff's business and did not know about Madoff's criminal activities.
Bonventre called Madoff "extremely intelligent" and noted his achievements in the industry, including his stint as NASDAQ chairman.
"He was generous, kind, considerate ... always seemed to have the right answer," said Bonventre.
He testified that Madoff had helped get his wife the best medical care when she had heart trouble and recounted other examples of his generosity.
"He appeared to be the most magnanimous, caring individual," said Bonventre. "... Amazing to watch, a very empathetic individual. At least that's how he appeared."
Now, he said, he realizes that Madoff "hurt so many people with what he did. There was no generosity spirit. None of that was real."
Bonventre said he started at the firm in 1968 as a clerk, rising to director of operations, a title he gave himself because customers wanted to know his role.
He said his side of the business had $700 million in assets, leading him to believe that Madoff had to be worth more than $1 billion.
In hindsight, he said, he suspects Madoff lied to him every day.