Such a delightful story! It's got a great mix of action and humour. I love the idea of a niffler bringing Luna and Rolf together.
Rolf is the perfect mix of disgruntled and soft-hearted. I loved the details that illustrated his love for (magical) animals despite his temporary rage at Crouton. His recollection of Crouton's run-in with Gringotts security was especially sweet. The contrast between his People Vest and Regular Vest -- love that the People Vest is the marked one -- was also a nice touch. And Crouton is an adorable imp! (Great name, also.)
One thing I especially liked about this story was what a vivid, concrete sense of place and time it had. Rolf's consideration of Diagon Alley shops through the lens of what a niffler might fancy was charming. (Also love that he gets the Firebolt's name wrong -- such an economical indication of his lack of interest in Quidditch!) And your descriptions of the twilit sky as reflected by shop windows and the marble columns of Gringotts and of closing time at Eeylops give the story a great sense of end-of-the-work-day urgency and fun.