She's always been a little flighty. Nothing holds her attention for very long, whether it be jobs, games, television shows, or even sexual partners. They all come and go, and she is flippant about the entire situation. The only thing that can attract or maintain her focus, to the best of her knowledge, is music.
He's an angel of the guardian class. He's watched over many a human, but he's been without a ward for a few years. He is suddenly given the task of watching over this human, but is warned there is something unusual about her and that Heaven's eye finds it difficult to follow her movements. He accepts the case, curious as to what would cause her to be so evasive to the watchers at the gate, and heads down to Earth.
For a bit, he's content to perform his job as he normally would, watching over her invisibly, in the corner of the room, a good few paces behind her as she walks...always hovering, never interacting.
Something begins to change shortly, however, and he is no longer content to watch. He takes on a human form, wanting to know her more personally.