Let me address what everyone has been saying, and say R.I.P. to the harem ending.
Now that I got that out of the way, I can now talk about what I'm thinking about who this person is. First off, lets name off the 2 people it likely can't be, which is Uruka and Furuhashi. Of course, I only said "likely" because their hairstyles doesn't match the silhouette at all, especially Uruka.
Next is the likely candidates, which are Rizu, sensei, and senpai. The reason for this is because they pretty much fit the silhouette's hairstyle. Rizu is most likely the candidate though because she has the first girl privilege and probably would be oblivious to the jinx in that situation.