Investigators still haven't determined why Kazmierczak, 27, opened fire in a lecture hall at his and Baty's alma mater, and she shed no light on a motive Sunday.
"The person I knew was not the one who went into Cole Hall and did that," Baty told CNN. "He was anything but a monster. He was probably the ... nicest, (most) caring person ever."
The day of the shooting or the day after, Baty received a package from Kazmierczak containing two textbooks, a cell phone and what she characterized as a "goodbye note."
"You've done so much for me," the note said, according to Baty. "You will make an excellent psychologist and social worker someday."
Another package contained a gun holster and ammunition. She confirmed that he had stopped taking an antidepressant about three weeks ago because "it made him feel like a zombie," but she denied that his recent behavior was unusual.
"He wasn't erratic. He wasn't delusional. He was Steve; he was normal," said Baty, who had turned down Associated Press requests by phone and in person for an interview.
Authorities have speculated that the couple might have split up just before the shooting. CNN said that during the interview televised Sunday, Baty described an on-off relationship and said she and Kazmierczak most recently had been living together.
"I still love him," she told CNN.
An NIU professor who befriended Kazmierczak and Baty during their years on campus told The Associated Press earlier Sunday that Baty was upset by media reports of their relationship as rocky and abusive.
Jim Thomas, an emeritus professor of sociology and criminology at NIU, said Baty feels she and Kazmierczak were a typical young couple.
"They were two people in love with all the pains, joys, squabbles, ups/downs of any other relationship," Thomas said.