Hilary Duff Mourns 'Lizzie McGuire' Co-Star Robert Carradine

Robert Carradine's family and former co-stars are paying tribute to the actor following his death at the age of 71.

Robert Carradine's manager and brother Keith Carradine confirmed his death Tuesday in a statement, telling NBC News that the actor, who was known for his roles in "Lizzie McGuire," "Revenge of the Nerds," and "Escape from L.A.," had "succumbed to" bipolar disorder after navigating the condition for nearly 20 years.

Tributes have since poured in following news of his death. Among those honoring his memory was Hilary Duff, who played Robert Carradine's daughter on "Lizzie McGuire."

"This one hurts. It's really hard to face this reality about an old friend," Duff wrote in an Instagram post.

"There was so much warmth in the McGuire family and I always felt so cared for by my on-screen parents," she continued. "I'll be forever grateful for that."

"I'm deeply sad to learn Bobby was suffering. My heart aches for him, his family, and everyone who loved him."

Jake Thomas, Robert Carradine's on-air son in "Lizzie McGuire," also took to Instagram to remember his former co-star.

"My heart hurts today. I was fortunate to know Bobby for most my life," wrote Thomas.

"And he was one of the coolest guys you could ever meet. Funny, pragmatic, sometimes cranky, always a little eccentric."

Thomas recalled "many fond memories being with him and his family" throughout his life.

"Good moments, challenging moments, and lots of laughs between. I looked up to him growing up," he wrote.

"And later in life, I came to realize he thought I was pretty neat, too. So I guess I was doing something right in his eyes."

Robert Carradine's niece, "The Goonies" actor Martha Plimpton, also wrote a tribute on social media.

"My Uncle Bobby was the best one of all 8 brothers; blood, adopted, and step," she wrote early Tuesday morning.

"Out of all of them, he was the absolute best. He was the one who cuddled me when I came to visit my father in LA for the first time at 8 or 9 years old," Plimpton continued.

"He cuddled and kidded me for being the only Carradine from New York. 'Come ova hee-ah! Maw-thuh! Get ova hee-uh!'"

"He did a terrible New York accent. I was an alien and Bobby joshed me about it, and I adored him for it," she wrote.

Plimpton went on to praise him as "the best actor of the bunch."

"He's honestly, in fact, the best actor of all of them," she wrote. "And I think I know why."

"Because he was KIND. He had inherent KINDNESS in him."

"He was dear, and loving, and nonjudgmental. And he loved honestly," added Plimpton.

"He made mistakes like anyone else, like any human. But he never lost his decency, or his heart."

Concluding, Plimpton wrote, "I love you, Bobby. You made my world happier. I will miss you enormously."

• Willie Colon, Architect of Urban Salsa Music, Dies at 75
• Alyssa Milano, Selma Blair, Ashton Kutcher, and More Remember Eric Dane, Dead at 53