When William meets Gráinne in the airport terminal, she says to him, “Fear not,” before kneeling to retie his wayward shoelace. Near the tale’s end she says to him, “Be not afraid…” The word choice is intentional, meant to convey that Gráinne is no mortal. Six times in the Holy Bible, angels utter those words when appearing before mortals.
Her retying of his shoelace alludes to William’s remark early on in the story that as long as he has unfinished business (he does not know how to tie his laces) he cannot die. In tying his lace, Gráinne finishes that business for him and henceforth he is vulnerable.