The actress took to Instagram to share a heartfelt note. "For Aaron — I'm deeply sorry that life was so hard for you and that you had to struggle in front of the whole world," the 35-year-old said. "You had a charm that was absolutely effervescent ... boy did my teenage self love you deeply. Sending love to your family at this time. Rest easy."
Of course, people thought the remarks were very kind. One person wrote, "This is so sweet. His episode of Lizzie McGuire was always my fav!" while another added, "Beautifully said."
A third person said, "This is beautiful. So very sad."
Duff and Carter previously dated when they were both 13 years old. The two were in on-again, off-again relationship until they split in 2003.
“I started dating Hilary [Duff] on my 13th birthday,” Carter, who was also linked to Lindsay Lohan, told CNBC in 2005. “I was dating her for like a year-and-a-half and then I just got a little bored so I went and I started getting to know Lindsay, dating Lindsay.”
“I’m really happy for her that she’s in a successful relationship and that she’s going to have a baby,” Carter said in 2012. “Sometimes I think to myself, maybe that could have been me but it is what it is.”
Two years later, he declared his love for the Disney Channel alum.
“I’ll spend the rest of my life trying to better myself to get back to her. I don’t care what ANY of you think,” the "I Want Candy" singer said.
Duff later responded to Carter's messages.
“I keep seeing that, that he keeps doing that. I don’t know how I feel. I mean, that was so long ago, and obviously, I’m still married, and I have a baby and we kind of just…” she said, adding that they didn't really "know each other."
As OK! previously reported, Carter, who shared son Prince with his ex Melanie Martin, was found dead in his California home.
"We are extremely saddened and shocked to confirm the passing of Aaron Carter today," his rep said in a statement. "At the moment his cause of death is being investigated. We ask that you give the family time and they will have more information when available. We cannot express the outpouring of love coming in."