I don't think it's a "who"; I think it's a "what" -- a spell, most likely. Probably the Cheering charm Hermione missed. I think she'll go to Flitwick to learn it in private so she can help Harry over Sirius. (Once she's had enough of him moping for 13 chapters...)

You know, you just might have something there. The first time I saw "Felix Felicis" I thought it sounded like a scientific name for some sort of plant or animal (like mimbulus mimbletonia). Maybe it's the scientific name for kneazle; Crookshanks is half kneazle, you know.