AceShowbiz -

Justin Bieber lost one of his biggest fans on Wednesday, September 25. 6-year-old Avalanna Routh, who's known by her family and friends as "Mrs. Bieber" due to her love for the Canadian singer, died after a courageous battle against an aggressive brain cancer.

"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," Bieber wrote a somber note after hearing the bad news. "RIP Avalanna. i love you. please show respect to her and her family. say a prayer for her and for them. i miss her."

The "Boyfriend" hitmaker posted a link to Cure ATRT, a website for a group that helps people who, like Routh, are diagnosed with atypical teratoid rhabdoid tumor. He additionally uploaded an Instagram picture of him with his late devotee. "My angel," so he called her.

Bieber made Routh's dream come true when he visited her ahead of Valentine's Day this year. They spent the day playing games, eating cupcakes, and exchanging autographs. "That was one of the best things I have ever done. She was AWESOME! Feeling really inspired now! #MrsBieber," he gushed back then.

Routh last tweeted about her idol on September 13 ahead of his "As Long As You Love Me" performance on "America's Got Talent" finale. "Trying to stay up late to see my husband on Americas Got Talent #supportingmyman #mrsbieber," she wrote.

Routh also thanked Jessica Biel for supporting Cure ATRT. The fiancee of Justin Timberlake replied, "You're my hero. It was an honor," and the young girl tweeted the actress back, "oh thank you..;) stay in touch ;) I wish I could have been there on Fri, great show;) #heart."