At least, the show made every indication that he's her father. All we know for certain is that she claims to be the biological daughter of Ellis Grey (Kate Burton) and that Ellis and Richard (James Pickens Jr.) had a years-long affair.
Anyway, we have a feeling we'll see a lot more of Maggie come Season 11. So we should use the intervening hiatus to acquaint ourselves with her portrayer, Kelly McCreary.
The Milwaukee-born 32-year-old is a Barnard College alum — according to Wikipedia, at least. Her most notable role so far was Tyra Dupre on The CW's Emily Owens, M.D., but she has also appeared in episodes of Castle, White Collar, and Rubicon. She also played the ill-fated double agent Clare on Scandal and has a part in the upcoming NBC conspiracy drama Odyssey.
On the big screen, Kelly has appeared in the films Being Flynn and How to Follow Strangers. She'll also play Eartha Kitt in the upcoming James Dead biopic Life — co-starring Dane DeHaan and Robert Pattinson.