The 23 year old, who was intellectually impaired, was over-friendly with associates who used her for sex or to transport drugs when she disappeared from regional Victoria while heavily pregnant 13 years ago.
"She'd give you the shirt off her back, that was the type of person she was," her mother Karen Fraser told an inquest on Monday.
"She didn't have a bad bone in her body, she was always doing nice things for everybody, how could somebody do something so bad to a pregnant woman?"
Police have long believed Krystal Fraser was murdered.
She discharged herself from a Bendigo hospital against medical advice on the night of June 20, 2009.
She jumped on a train, telling people she was going to a party in the town of Cohuna, but she never made it there.
Ms Fraser received a 40-second phone call from a public phone at 11.59pm on the night she disappeared.
Almost three hours later, her mobile phone was pinged by a tower in Leitchville, 27km from the town of Pyramid Hill where she was last seen.
Investigators say there may have been a confrontation between Ms Fraser and the father of her unborn child before her disappearance.
It's believed her final phone call was from that man.
The inquest into Ms Fraser's disappearance will examine her believed final movements and the circumstances of her possible death.
More than 20 witnesses will give evidence over six days of hearings.
Coroner Katherine Lorenz said there was no evidence to suggest Ms Fraser or her baby were alive today.
"There is no plausible explanation that I've been able to find why Krystal has not made contact with her family for 13 years, in circumstances where she appeared to have been excitedly looking forward to the birth of her son," she said.
Karen Fraser believes her daughter, who vanished about three days before her due date, may have been murdered by the father of her child.
"Maybe she told someone they were the father and things got out of control and somebody accidentally hurt her," she said.
She said Ms Fraser told her she did not know who the father was, but had contemplated doing a DNA test.
"She said she had a couple of different partners at the same time but she's not sure," Karen Fraser said.
Her father, Neil Fraser, said she attracted "undesirables" who had no work and used her to buy drugs or cigarettes since she received a disability pension.
"If they were the father, they wouldn't have done anything for her," he said.
"They would have looked at the pension - she's getting money, we'll have that money."
There is a $1 million reward out for information on Ms Fraser's disappearings and a person was arrested and interviewed in 2018 but no charges were laid.
The inquest continues.
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