Wladimir Klitschko is mourning his former fiancée, actress Hayden Panettiere, stating she remained an "important part" of his life and vowing to keep her memory alive for their daughter, Kaya.

"Our family is going through a time of profound shock and grief," the Ukrainian former heavyweight boxing champion wrote on Instagram following Panettiere’s death at 36. "She left this world far too soon."

Klitschko, 50, acknowledged that although Panettiere was "no longer my partner," their shared history and her place in their lives could not be erased. "To our daughter Kaya, I will always speak of her mother with respect and make sure she remembers the person she was," he added.

Panettiere died Sunday in South Carolina. The Greenville County Coroner’s Office and Greenville City Police Department are investigating her death. Preliminary autopsy results have shown no signs of trauma contributing to her death, with the cause and manner of death pending.

In 2018, Panettiere transferred custody of Kaya to Klitschko, explaining at the time that she was in a difficult place and wanted to protect her daughter. "I just knew that if I wanted to... I knew how bad a place I was in," she told Fox News Digital. "I hid it from my daughter as much as I could."

Panettiere had publicly discussed her struggles, including the effects of growing up in the spotlight, alcohol addiction, and postpartum depression after Kaya's birth in 2014, detailed in her memoir "This Is Me." She expressed concern that a public custody battle between her and Klitschko would harm their daughter. "When I saw her in the environment over there [in Europe] and the support system that she had, I said to myself, ‘If I fight, if I go to war about this, it’s going to permanently damage my child,’" she recalled. "It’s going to be incredibly public. She’s got two very famous parents, and it’s going to be ugly."

Ultimately, Panettiere decided Kaya should remain with Klitschko for her safety and well-being, while she planned to visit as much as possible. "I said, ‘You know what? Let her grow up here,’" she remembered. "‘I will travel here as much as I can. I know she’s safe. I know she’s well taken care of — beyond well taken care of. She has an incredible life. And I also knew that there would be a day when she would come back to me.’"

In his tribute, Klitschko also recognized Panettiere's career, writing, "She built an incredible career through immense talent, while also facing the darker sides of a very demanding industry." He offered condolences to her family and friends and requested privacy.

First responders found Panettiere in cardiac arrest, and EMS personnel attempted resuscitation before she was pronounced dead. The investigation into her death remains active.

Panettiere began her acting career as an infant and voiced Princess Dot in Pixar’s "A Bug's Life." She also appeared in "Remember the Titans," "Ice Princess," and "Bring It On: All or Nothing." She gained significant recognition for her role as Claire Bennet in the NBC series "Heroes" and later starred as Juliette Barnes in "Nashville" from 2012 to 2018.