Avalanna Routh, the 6-year-old girl who captured the hearts of Justin Bieber and his fans who called herself “Mrs. Bieber,” died after she lost her battle with a rare brain cancer.

“Our darling Avalanna went to Heaven this morning,” said the official Twitter account of the Merrimac, Mass., girl. “Oh Avalanna, the brightest star -- you took our hearts with you, our greatest Love.”

As Avalanna was fighting her brain cancer, Bieber helped the 6-year-old’s dreams come true by visiting her and even having her come up on stage at his concerts, singing “One Less Lonely Girl” to “Mrs. Bieber.”

Bieber and Avalanna also held a mock wedding in which “Mrs. Bieber” showed off a fake wedding ring.

In a Twitter post after their February 2012 “marriage,” Bieber wrote, "That was one of the best things i have ever done. she was AWESOME! Feeling really inspired now! "#MrsBieber really inspired me."

Avalanna’s mother, Aileen Routh, told the Boston Globe how touched she felt after Bieber held the fake wedding for her ailing daughter.

“We were Bieber fans before, but there is now a whole new respect for this kid that did such a great job with her and was just so graceful in doing it,” she told the paper.

“Avalanna means the world to me,” Bieber said in June in a video interview, which you can also view below.

“She’s one of my biggest fans. She has a rare condition where she’s not going to make it much longer,” the singing sensation said as he was overcome with emotion. “It kind of hurts sometimes that I feel like I might not be able to see her again.”

Cameron Routh, Avalanna’s father, said his daughter first met Bieber around Valentine’s Day.

“Avalanna’s relationship with Justin is very natural. It’s very natural to see them together. It’s as if there’s no one else around and she doesn’t look at Justin as a superstar – it’s just a friend of hers,” Cameron Routh said in the same interview.

“Every time I see her I just try to make her as happy as possible,” Bieber said.

Avalanna’s plight was followed by millions of Beliebers, who expressed their condolences on Twitter after learning of the 6-year-old’s death from brain cancer.

“RIP Avalanna” was trending on the micro-blogging site Wednesday.

"The world lost a fighter, but heaven gained an Angel,” wrote the @OfficialCANCER Twitter account. The message was retweeted more than 11,000 times.

“RIP Avalanna. Rest In Peace little Mrs Bieber,” wrote @TeamofBieber.

“RIP Avalanna, Justin & Beliebers will always love you. Our beautiful angel,” said @BieberCore.

“Jazzy lost her bestfriend, Justin lost his wife, Parents lost there child, we lost a sister and heaven gained an angel :'( RIP Avalanna,” said @BoyyBieberCrew.

“Avalanna always made Justin so happy. I can't believe she's gone. RIP Avalanna, our princess.Gone, but never forgotten,” wrote @iBelieBinJus10.