Who says, "I am one of the few honest people that I have ever known." What caused the person to say this?

Nick:

Every one suspects himself of at least one of the cardinal virtues, and this is mine: I am one of the few honest people that I have ever known.

When Jordan comments that she like Nick because he "isn't careless", a lightbulb goes off. He comes to understand that he cannot pursue a relationship with Jordan while he is carelessly leading a girl on from back home.

Her gray, sun-strained eyes stared straight ahead, but she had deliberately shifted our relations, and for a moment I thought I loved her. But I am slow-thinking and full of interior rules that act as brakes on my desires, and I knew that first I had to get myself definitely out of that tangle back home. I’d been writing letters once a week and signing them: “Love, Nick,” and all I could think of was how, when that certain girl played tennis, a faint mustache of perspiration appeared on her upper lip. Nevertheless there was a vague understanding that had to be tactfully broken off before I was free.