You have asked a really interesting question as there is a certain ambiguity about the character of Hapsy in this excellent short story. Hapsy is an ambiguous character because her identity and whereabouts are open to interpretation. Her name suggests that she had or has a sunny disposition and that she made or makes Granny Weatherall happy. Note how Hapsy is referred to in the last paragraph:
You'll never see Hapsy again. What about her? "I thought you'd never come." Granny made a long journey outward, looking for Hapsy. What if I don't find her? What then? Her heart sank down and down, there was no bottom to death, she couldn't come to the end of it.
Of course, it is vital to recognise that the stream of consciousness narration means that everything is very confused as Granny Weatherall moves closer and closer to death and makes more and more random connections between her scattered thoughts. And yet it is possible to suggest that Hapsy could be her favourite daughter that she longs to see one more time, or perhaps that Hapsy is a character or a dear friend who is going to welcome Granny Weatherall into the afterlife.
Either way, Hapsy does not appear, and perhaps this is another sign of the rejection of the jilting that Granny Weatherall so courageously faces at the end of the story as she yields herself up to death at the very end.