Mr. Behrman was apparently intended to be the antithesis of "a knight on a white horse." In this story, "a knight on a white horse" would probably be a romantic young unknown artist who sacrifices his life to save Johnsy, the girl he loves. O. Henry wanted to write a story about a girl who was dying of pneumonia and had the conviction that she would die when the last ivy leaf on the wall of a nearby house fell. The author's plot was based on the idea that someone might paint a fake ivy leaf on the wall in the middle of the night, and the girl would recover because of the example set by what appeared to be one brave remaining ivy leaf. O. Henry wanted his ending to come as a surprise, which was his trademark as a writer. O. Henry had to introduce a painter into the story who would end up being the one to paint the fake leaf and sacrifice his life in doing so, but he didn't want the reader to suspect this ending when he introduced the painter. Therefore, O. Henry did not want a man who might conceivably be in love with the sick girl, because right away the reader would say, "Ah, ha! He's going to save her somehow!" So O. Henry needed a man who was a painter and who cared for Johnsy but who could not be imagined as her lover. This explains why Sue has the following interchange with the doctor:
“She—she wanted to paint the Bay of Naples some day,” said Sue.
“Paint?—bosh! Has she anything on her mind worth thinking about twice—a man, for instance?”
“A man?” said Sue, with a jew's-harp twang in her voice. “Is a man worth—but, no, doctor; there is nothing of the kind.”
Then when O. Henry introduces Behrman, there can be no suspicion that this old man could be Johnsy's potential savior. Behrman is old. He is a drunkard. He seems half-crazy. He smells of gin. He has a bad temper. In his broken English, he speaks contemptuously of Johnsy's idea that she is going to die when the last leaf falls.
“Vass!” he cried. “Is dere people in de world mit der foolishness to die because leafs dey drop off from a confounded vine? I haf not heard of such a thing. No, I will not bose as a model for your fool hermit-dunderhead. Vy do you allow dot silly pusiness to come in der prain of her? Ach, dot poor leetle Miss Yohnsy.”
Behrman's character and personality are intended to mislead the reader. He seems angry, obstinate, and self-centered. His broken English is a smokescreen to hide his thoughts and feelings. He doesn't even seem physically capable of climbing a ladder with paints, brushes, and a lantern to paint a leaf on a wall twenty feet above the ground. It turns out, of course, that Behrman is a romantic at heart and the kind of man who actually would risk his life to paint a leaf on the wall of the nearby building. The reader is taken by surprise when this fact is revealed at the very end when Sue tells Johnsy:
"Didn't you wonder why it never fluttered or moved when the wind blew? Ah, darling, it's Behrman's masterpiece—he painted it there the night that the last leaf fell.”