In the character descriptions preceding the play, Jim is described as a "nice, ordinary, young man." He is the emissary from the world of normality. Yet this ordinary and simple person, seemingly out of place with the other characters, plays an important role in the climax of the play.
The audience is forewarned of Jim's character even before he makes his first appearance. Tom tells Amanda that the long-awaited gentleman caller is soon to come. Tom refers to Jim as a plain person, someone over whom there is no need to make a fuss. He earns only slightly more than does Tom and can in no way be compared to the magnificent gentlemen callers that Amanda used to have.
Jim's plainness is seen in his every action. He is interested in sports and does not understand Tom's more illusory ambitions to escape from the warehouse. His conversation shows him to be quite ordinary and plain. Thus, while Jim is the long-awaited gentleman caller, he is not a prize except in Laura's mind.
The ordinary aspect of Jim's character seems to come to life in his conversation with Laura. But it is contact with the ordinary that Laura needs. Thus it is not surprising that the ordinary seems to Laura to be the essence of magnificence. And since Laura had known Jim in high school when he was the all-American boy, she could never bring herself to look on him now in any way other than exceptional. He is the one boy that she has had a crush on. He is her ideal.
In the candlelight conversation with Laura, he becomes so wrapped up in reliving his own past that he seems once again to think that he is the high school hero who swept the girls off their feet. He becomes so engrossed in the past that he not only breaks Laura's favorite piece of glass, but he also breaks Laura's dreams and hopes. He was so engrossed playing the role of high school hero and amateur psychiatrist that he failed to see what emotions he was building up in Laura. His most accurate description of himself is when he refers to himself as a "stumblejohn."
But Jim's function in the play is more important then his seemingly ordinary character would allow. Since Laura lives in a world of illusion and dream, Jim, as the ordinary person, seems to Laura to be wonderful and exceptional. He is so different from her own world that he appears to be the knight in shining armor.