Stella is Arthur's first wife. Throughout his adventures in Crythin Gifford (if you can call them that), she's referenced as a refuge, something for him to look forward getting back to.
But my main sensation was one of tedium and a certain lethargy, combined with a desire to finish the job and be back in London with my dear Stella. I remembered that I meant to tell her that we should get a small dog, as like Spider as possible, once we had a house of our own. (9.18)
For Arthur, Stella represents everything that's safe and normal. He's convinced that when he returns to her and resumes his normal life, all the unpleasantness that he collected at Eel Marsh House will wash off.
But for us, Stella represents the fact that the past can reach out wherever you are—including where you feel most secure. And it's bad. She lingers on for ten months, with Arthur mourning the loss of his son and watching her die. Poor Stella: she's sacrificed for the revenge of a woman she's never met.
But isn't that just the way the past works sometimes?