The 20-year-old star became friends with Foster, 16, when she arranged the band's meeting with an eight-year-old cancer patient, reported Daily Mirror.
"Yes, Niall and I have been talking in private. He's the most famous person in my phone after Nicole Scherzinger. I hope he fancies me - he's cool," Foster said.
The duo apparently also met backstage recently.
"I also watched the boys do a run-through of their songs, and Niall and I started messing around in the corridor backstage," she said.
"Niall tweeted me when we first met but now we just direct message," Foster said.