Fontaine's body was found wrapped in a bag in the Red River near the Alexander Docks on Sunday, more than a week after she was reported missing in the city.
Winnipeg police are treating her death as a homicide, but they have not released any details about how she died or what may have happened to her during the time she was missing.
"It's like somebody just tore my heart out and they just keep stomping on it," Thelma Favel, Fontaine's great-aunt, told CBC News.
"Why would someone do this to a baby?"
Police officers were going door to door in the inner city on Wednesday, asking residents if they have noticed anything suspicious recently.
The teen's death has sparked an outpouring of grief and renewed calls for a national inquiry into missing and murdered aboriginal women and girls.
Father beaten to death in 2011
Fontaine grew up in the Sagkeeng First Nation, located northeast of Winnipeg.
Favel said she looked after Tina and her sister for the past 11 years, while their father battled cancer.
Eugene Fontaine, 41, was found beaten to death on Oct. 31, 2011. Two men later pleaded guilty to manslaughter.
Favel said she went to various Child and Family Services agencies to get counselling for Tina, who she said had trouble dealing with her father's killing.
"I [sought] help from all agencies, and I was just turned away," she said.
Tina recently met her biological mother, who Favel claims introduced the teenager to drugs.
Favel said she last saw Fontaine on July 1, when the teen ran away from Sagkeeng.
The great-aunt said she contacted a CFS agency that located Fontaine and brought her into a voluntary placement at a foster home in Winnipeg.
It's not clear when Fontaine entered the foster home or how long she stayed there. She was reported missing on Aug. 9.
"They had her in a foster home and it took them three weeks to tell me that she was AWOL," Favel said.
Spent time downtown
Police investigators said Fontaine was known to spend time in downtown Winnipeg, near the Portage Place shopping centre.
Lana Fontaine, Tina's aunt, said her niece would go to her Winnipeg home whenever she ran away. But on Sunday, police officers appeared on her doorstep instead.
"They told me that they found her and it wasn't good," she said, sobbing. "I just knew in my heart she was gone."
Police said Fontaine's body was discovered while divers were searching for a man who had been seen struggling in the water near The Forks on Friday.
Since she was under the care of Child and Family Services, her death is automatically being reviewed by Manitoba's Office of the Children's Advocate.
Lana Fontaine, who last saw her niece a week before she disappeared, said Tina wanted to find a job and finish school.
"She had such good plans," she said.
"She was a beautiful, sweet girl. All she wanted was to live. Whoever did this to her, please come forward and let her rest."