Los Angeles, California - Billie Eilish is opening up about her important friendship with fellow pop star Justin Bieber.

On Monday, the 21-year-old appeared with her brother Finneas on the podcast Conan O'Brien Needs A Friend, where she dished on her close relationship with the Love Yourself singer.

Eilish told O'Brien she's "so grateful" that Bieber is in her life, sharing that they've leaned on each another while navigating the challenges of fame and the music industry.

"I am so so thankful for him and like the way that he was to me when I needed it, and he needed it," the Ocean Eyes singer said.

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Eilish said that while she may have started as a die-hard Belieber, their bond has gone far deeper than mutual appreciation of each other's work.

"He just doesn't give up on me, and I know that sounds stupid, but it's really true," Eilish said. "He really makes me feel so loved and seen, and he's always reaching out to me in the sweetest ways, in the most comforting ways of just like 'you're not alone in this, I was there.'"

Their similar experience in finding success at a young age has made Bieber want to be the role model he didn't have at Eilish's age, and she recalled a recent "heartbreaking" moment when the 29-year-old revealed just how challenging his early days were.

"He said something that was so heartbreaking, which was like, 'I'm just really glad that honestly you have me to talk to about this because I actually understand, and there's really almost nobody in the world who can know what it was like except kind of you and me and a few others,' Eilish said.

Bieber also told her that he wished he had someone to guide him when he was her age because he was "alone" while navigating the start of his career.

Aside from their personal support of each other, their friendship has led to some impressive moments professionally as well.

The pair previously collaborated on a hit remix of Eilish's Bad Guy that featured a new verse from Bieber.