Magdalena SzablinskaIt, 16, suffered a cardiac arrest as she waited for a bus outside St Mark's Catholic School in Hounslow.
One Direction fans who had befriended Magda at gigs and online flooded Twitter and other social media networks to pay their respects, and on Friday, Styles added to the tributes with a tweet.
He wrote: "It's incredibly sad to hear the news about Magda. She was so so lovely, i am thinking of her and her family. RIP."
The pop star had met with the teenager, who ran fansite @1DCrew on Twitter, backstage at gigs and one photograph of her with her hero appeared in newspapers and on websites reporting the news of her death.
Her father Tadeusz tells the Evening Standard: "She was an angel and everybody has said so. So many people have contacted us to pay their respects, she was so loved.
"She liked One Direction and had so many friends on Facebook... She was very popular and doing very well, she was our star."