Avalanna Routh, the 6-year-old Justin Bieber fan also known as “Mrs. Justin Bieber,” has died from a rare form of brain cancer.

On Wednesday, Routh's parents posted the tragic news on her Twitter account, writing:

Last Valentine's Day, Routh's wishes came true when she spent the day with Bieber after he caught wind of her Facebook campaign to meet her idol, which amassed more than 12,000 supporters. He then retweeted the link to his millions of followers and invited Routh and her family to spend Monday with him in Manhattan.

During their day together, Routh played board games with the pop star, kissed him on his cheek and was granted what was quite possibly the best gift of all -- getting to mess with Biebs' hair.

"That was one of the best things I have ever done. she was AWESOME. Feeling really inspired right now! #MrsBieber," the singer later tweeted, following up with a photo of him kissing a clearly ecstatic Routh.

Routh was diagnosed with ATRT, an extremely rare form of brain cancer that affects fewer than 30 people per year. According to her mother, Aileen Routh, she had been fighting the disease for five and a half years.

Months after meeting Bieber, Avalanna was still a huge fan, and on Sept. 13 she even tweeted:

"@justinbieber Trying to stay up late to see my husband on Americas Got Talent #supportingmyman #mrsbieber"

After learning the news of his young fan's passing, Bieber wrote on Twitter:

"just got the worst news ever. one of the greatest spirits i have ever known is gone. please pray for her family and for her. RIP Avalanna. i love you. please show respect to her and her family. say a prayer for her and for them. i miss her."

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  • NEW YORK - SEPTEMBER 01: Musician Justin Bieber visits the Nintendo World Store on September 1, 2009 in New York City. (Photo by Bryan Bedder/Getty Images)

  • NEW YORK - NOVEMBER 15: Justin Bieber performs on ABC's Good Morning America' at ABC News' Good Morning America Times Square Studio on November 15, 2009 in New York City. (Photo by Michael N. Todaro/WireImage)

  • NEW YORK - DECEMBER 11: Justin Bieber attends the Z100s Jingle Ball 2009 presented by H&M at Madison Square Garden on December 11, 2009 in New York City. (Photo by Jim Spellman/WireImage for Clear Channel Radio)

  • LOS ANGELES, CA - FEBRUARY 13: Singer Justin Bieber poses after his free performance presented by KIIS-FM at Nokia Plaza L.A. Live on February 13, 2010 in Los Angeles, California. (Photo by Angela Weiss/Getty Images)

  • NEW YORK - NOVEMBER 26: Justin Bieber promotes his new book 'First Step 2 Forever' at Barnes & Noble, 5th Avenue on November 26, 2010 in New York City. (Photo by Andy Kropa/Getty Images)

  • LOS ANGELES, CA - MAY 15: Justin Bieber attends KIIS FM's 2010 Wango Tango Concert at Staples Center on May 15, 2010 in Los Angeles, California. (Photo by Jason LaVeris/FilmMagic)

  • Teen idol Justin Bieber arrives on the red carpet for the 68th annual Golden Globe awards at the Beverly Hilton Hotel in Beverly Hills, California, on January 16, 2011. AFP PHOTO/Valerie MACON (Photo credit should read VALERIE MACON/AFP/Getty Images)

  • LONDON, ENGLAND - FEBRUARY 16: Justin Bieber attends the 'Justin Bieber: Never Say Never' premiere at the O2 Cineworld on February 16, 2011 in London, England. (Photo by Jon Furniss/WireImage)

  • LAS VEGAS, NV - MAY 22: Singer Justin Bieber poses in the press room with the Digital Artist of the Year award during the 2011 Billboard Music Awards at the MGM Grand Garden Arena May 22, 2011 in Las Vegas, Nevada. (Photo by Isaac Brekken/Getty Images)

  • NASHVILLE, TN - JUNE 08: Justin Bieber attends the 2011 CMT Music Awards at the Bridgestone Arena on June 8, 2011 in Nashville, Tennessee. (Photo by Jeff Kravitz/FilmMagic)

  • Singer Justin Bieber attends the 2011 BET Awards at The Shrine Auditorium on June 26, 2011 in Los Angeles, California. (FilmMagic)

  • UNIVERSAL CITY, CA - AUGUST 07: Singer Justin Bieber arrives at the 2011 Teen Choice Awards held at Gibson Amphitheatre on August 7, 2011 in Universal City, California. (Photo by Jon Kopaloff/FilmMagic)

  • Singer Justin Bieber arrives at the The 28th Annual MTV Video Music Awards at Nokia Theatre L.A. LIVE on August 28, 2011 in Los Angeles, California. (WireImage)

  • NEW YORK, NY - SEPTEMBER 08: Justin Bieber attends the Dolce & Gabbana Boutique on September 8, 2011 in New York City. (Photo by Eugene Gologursky/Getty Images for Dolce & Gabbana)

  • Singer Justin Bieber poses in the press room at The 53rd Annual GRAMMY Awards held at Staples Center on February 13, 2011 in Los Angeles, California. (Photo by Michael Tran/FilmMagic)

  • ATLANTA, GA - SEPTEMBER 17: (EXCLUSIVE COVERAGE) Musician and 2011 Horizon Award Recipien Justin Bieber poses backstage at the 33rd Annual Georgia Music Hall Of Fame Awards at the Cobb Energy Performing Arts Center on September 17, 2011 in Atlanta, Georgia. (Photo by Ben Rose/WireImage)

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