Harlow's Links
Harlow did not initially gravitate towards Darcy, but rather her vicinity. He noticed a gathering of humans in that area and decided he would stay near the area too. Quickly Harlow found that he was not welcome and was considered a loiterer--that's when Darcy took him in. Everything he knows about socially acceptable behavior, clothes, and technology he learned from her, and although it doesn't often show through his aloofness, he very much cares for her.
Though Darcy is far from the nurturing type, she was pressed into providing Harlow temporary shelter after she watched him and realized he had no idea how to function in society. This "temporary" shelter turned into something more long-term, and even extended into Darcy considering Harlow somewhat of a friend. Even after months of knowing him she has no clues to his origins, but she doesn't press.