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When Bieber heard wind of the campaign that garnered over 12,000 supporters, the pop star invited Routh and her family to spend the day with him in Manhattan for a special Valentine's Day date.
The girl, who called herself "Mrs. Bieber" having held her own adorable wedding to the singer, played board games with the star, kissed him on the cheek and even got to touch Bieber's headline-making hair.
Bieber invited the peppy and courageous Routh back to New York for an appearance on the Today Show and brought her on stage with him at the Apollo Theater last June.
"RIP Avalanna" was trending on Twitter worldwide this week. Justin Bieber tweeted about Routh on Wednesday, saying: "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. (sic)" Bieber also tweeted a link to the foundation her family works with that helps those with her rare condition.
Routh was diagnosed with atypical teratoid rhabdoid tumor, a rare form of brain cancer that has only around 30 diagnoses in the U.S. each year.
Tiffany Lee writes for Yahoo! Music