While a case could be made that either Irene or Clare is the protagonist of Passing by Nella Larsen, it seems clear to me that Irene is the character in this story who eventually comes to a new understanding about her world and about human nature.
Clare is perhaps the more interesting character, as she is the one who is living a lie. She is the black woman who is light-skinned enough to live her life "passing" as a white woman; even more, she is a risk-taker who likes to live her life dangerously.
The trouble with Clare was, not only that she wanted to have her cake and eat it too, but that she wanted to nibble at the cakes of other folk as well.
Clare lives her life as a fraud and a user. While there is nothing that says the protagonist has to be a "good guy," Clare is too self-absorbed and selfish to really admit her flaws, something she must do before there can be any believable growth or change.
Irene is the relatively steady and stable character of the two women. It is through her that we get our first impressions of Clare--even before we actually meet her--and what we see is not pretty. First we see Clare as Irene sees her, and then we meet her for ourselves and know that Irene's perceptions of Clare are accurate. Without Irene, we have no reason to even meet Clare.
Unfortunately, Irene is ultimately too kind to avoid becoming victimized by Clare. Irene does not want to do the favor Clare asks her to do both for her own reasons and to spare her friends from Irene's condescending ways and from being used by her. She avoids the phone calls for as long as she possibly can, but eventually Clare wears Irene down and gets her way.
Irene's visit is just as bad as she anticipated it would be, as Clare's husband makes a vulgar display of his prejudice. When Irene leaves, she blames herself, not Clare, for the disastrous meeting. Irene knew what this meeting would be like and she chose to come, so she does not blame the conniving woman. This is a rather heroic position to take and exhibits some of the growth Irene undergoes through the course of this novel. Notice that Clare remains static.
When Clare writes Irene another letter, two years later, she asks (demands) another favor, to meet "with her own people"; however, Irene remembers that when Clare had a chance to speak up against her husband's vocal bigotry she was silent, and Irene has learned enough to avoid Clare this time--another sign of growth.
The story progresses and the entanglements increase. Clare has somehow managed to insinuate herself into Irene's life, even to the point of having an affair with her husband, Brian. Though Irene has several opportunities to expose Clare as a fraud both to her racist husband and to the black community, she does not do it. Even though she has been betrayed, she takes the higher road and does not do the betraying. What she does do is wish that Clare were dead, a wish that does come true.
There is some doubt about Irene's role in Clare's death, but a consistent reading of Irene throughout the novel would suggest that, in the end, she did not follow through on her desire to have Clare dead.
This is the story about Irene's loss of innocence. She is a dynamic character who learns, the hard way, that a person who lies about one thing will have no compunctions against lying about everything. It is a hard lesson, and Irene will undoubtedly suffer from this knowledge for the rest of her life. This is not Clare's story; it is Irene's.