Obi first meets Clara at a dance in England. She is so beautiful that he becomes awkward around her and continuously steps on her toes; she doesn't dance with him again. Then, by coincidence, they both take the same boat home from England to Nigeria.
As a nurse, Clara goes around administering medicine to seasick patients, and she seems like an angel to Obi. He quickly initiates a relationship. Clara is a practical girl, not to mention smart and hard working. Though she is Igbo, she is educated in the Western tradition and lives a modern lifestyle. However, Clara seems to lack the disdain Obi has for traditional culture. Every fault Obi has, Clara counter-balances.
There has be something wrong with somebody who seems too perfect, right? Clara was right when she first told Obi that he didn't really love her and she would regret giving in to his flattery. Soon we discover she has a secret. Her secret is culturally specific – something only another Igbo would care about. She is an osu, part of a social caste forbidden to marry because one of her ancestors was dedicated to a god. They are considered taboo. As Isaac points out, marrying her would be like marrying a leper.
Once Clara admits this to Obi, she tries to break up with him. She says it's not fair to keep the relationship going; she doesn't want to come between Obi and his family. Though Clara was educated in England, and probably wonders about the justice of her taboo status, she never voices those thoughts aloud. Instead, it's clear that she understands how deeply these traditions run. She knows that Obi's parents will never accept her as a daughter-in-law, even though they are Christians and should, theoretically, disagree with a tradition like this. Clara's parents are Christians, too, but that hasn't changed their status as osus or the fact that they are outcasts, even in the church. Though Clara keeps trying to break up with Obi because of this fact, and Obi keeps stubbornly refusing to be broken up with, the relationship's demise is clear, at least to her. She's just sorry, in the end, that she didn't do something about it before she ended up pregnant. That is what makes her bitter.
When Obi returns from his family's house after breaking the news that he is engaged to an osu, Clara understands that their relationship is over, even though Obi only tells her that they should lay low for a while. She could handle the breakup when she knew it was about family, but after the abortion, she isn't interested in seeing Obi ever again.
Though Clara allows Obi to arrange the abortion for her, it's not clear if she wants to go through with it. However, it's obvious that she doesn't forgive him for it. When the doctor suggests that they get married instead, Clara says, sullenly, "I don't want to marry him" (15.50). By then, she's figured out or decided that Obi is not a great guy. The abortion goes badly and Clara spends weeks in the hospital recovering. Though Obi tries to get back together with her, sending her a note saying they can work this out, Clara returns the note unopened.