Hello! Secondary characters contribute to the characterization of the protagonist by allowing the reader to compare and contrast between the main and secondary characters. Take Wilson, for example. He is emotional, prone to hot outbursts, and fancies himself the better man for Louise. He imagines that his love for poetry will help him connect to "Literary Louise," as the officers mockingly call her. Yet, Harris tells Wilson that if he "had a wife like that, I'd sleep with n*****s too." Wilson himself is no match for Scobie's cool composure, for despite his shortcomings, Scobie's main goal is the happiness of his wife.
"What would you do if I told her everything about Mrs. Rolt?"
"But you have told her, Wilson. What you believe. But she prefers my story."
"One day I'll ruin you, Scobie."
"Would that help Louise?"
The portrayal of Louise as a pompous bore and of Wilson as an emotionally-unstable foe reinforces our understanding of the protagonist's state of despair and pessimism. He cannot make such a self-absorbed woman happy and he definitely cannot plead his case to such an unreliable policeman. Wilson would never understand why he borrows from Yusuf, and he wouldn't excuse it either.
Both Louise and Helen make their own set of demands on Scobie. These two characters clarify for the reader that pity, guilt, and responsibility are insufficient contributions to lasting satisfaction in any relationship. The lack of honesty underpins each of Scobie's interactions with both women. These interactions pinpoint the protagonist's moral ambivalence and his dysfunctional relationship with God. Scobie rationalizes that since he cannot trust God, he will do what he has to do to secure the happiness of those who inspire his pity and his guilt. He reasons that killing himself is the only way he knows of to atone for what he thinks is his unworthiness before God.
And you too, God you are ill with me. I can't go on, month after month, insulting you.
You'll be better off if you lose me once and for all.
I am going to damn myself, whatever that means. I've longed for peace and I'm never going to know peace again. But you'll be at peace when I am out of your reach.
Graham Greene allows Scobie stream-of-consciousness monologues/interior dialogues to provide us a glimpse of his protagonist's personal anguish and torment. Greene uses dramatic irony to illustrate that Scobie's suicide will not bring about true happiness for Helen and Louise; we already know this, but Scobie didn't. Indeed, after the suicide, Louise is angry with Scobie for messing things up so badly, and also for possibly destroying any chance of redeeming his soul for all eternity. She remains just as contemptuous and oblivious to her husband's private anguish. Helen herself takes up with Bagster after Scobie's suicide; her own private struggles with the reality of God's existence compels her to move through life on a sort of autopilot (as Bagster accuses her of doing when he tries to bed her) in order to assuage her pain. Thanks for the question.