Of the three main characters, Stella is the least developed. She doesn't have any dialogue of her own and we see her only through three basic snippets.
1: Leo's first impression of her, when "He had a vivid impression that he had met her before, but try as he might he could not place her although he could almost recall her name" (143).
2: Salzman's condemnation of her: "For her to be poor was a sin. This is why to me she is dead now" (184).
3: Stella's own feelings, very briefly described at the very end, when "her eyes—clearly her father's—were filled with desperate innocence" (201).
Let's deal with #1 and #2 first. The story initially presents Stella as a holy savior and then as a wicked strumpet. This is known as the virgin/whore dichotomy. If she ain't a sweet innocent, she's a fallen woman. It seems impossible for this girl to be full of both sugar and spice.
We meet Stella as presented by two men who need her to be what they want, rather than who she is. She's essentially the prize Leo has been seeking: a romantic interest who can make him happy and ultimately heal his spiritual wounds. Her father has disowned her because she was too wild, probably in a sexual way.
But we do finally get some sense of who she really is at the very end, with those haunted eyes and the way she waits for Leo "uneasily and shyly" (201). Suddenly her character gets a lot more interesting than being just an angel or a harlot. She's suddenly human: a woman who has been hurt, who has rebelled against the forces of authority (i.e. Daddy), and who is looking for a way to redeem herself.
Wow. Maybe she is Leo's soul mate. Leo's intensity in his search for salvation may be matched by hers': she needs to be saved as much as he does. She may very well be an angel, albeit one who has seen too much of life and is a little tattered round the feathers. But to her, Leo might look like an angel too… perhaps both of them were sent to save each other from lives of loneliness?