Why do you think William Sydney Porter built the character of Mister Behrman as a pathetic old artist?

O. Henry was was gifted with a rich imagination. In his story "The Last Leaf," he may have built the whole plot around a single ivy leaf he noticed still clinging to a vine on the side of a brick wall. If so, he would have had to invent characters to suit the needs of his plot. He probably felt he had to introduce the painter who would eventually paint an ivy leaf on the wall of a nearby brick building in order to keep a girl from dying. O. Henry might have considered it too obvious to invent a young painter who was in love with Johnsy and sacrificed his life for her. The author quite intentionally dispels that possibility in the conversation between the doctor and Sue.

"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.”

So there is "nothing of the kind" in Johnsy's life--but still the author needs a painter. So he introduces a man he calls Old Behrman. The reader will not suspect that this character would risk his life to paint an ivy leaf on a wall. For one thing, he is too old to climb a twenty-foot ladder. He hasn't painted anything in years. He is drunk on gin all the time. And most of all he pretends not to be sentimental and dismisses the notion that anybody could die just because a leaf falls from a vine.

“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.”

It would spoil O. Henry's ending if the reader suspected that this funny old man might get the idea of painting a leaf on a wall. The fact that Old Berhman speaks such heavily accented English also helps to disguise his thoughts and intentions. But the factors that would appear to work against his deciding to paint a last leaf for Johnsy could also work in favor of his actually doing it. He is too old--but his life is all behind him, so what has he to lose? He is a heavy drinker--but being drunk might explain why he gets such a wild idea. He says Johnsy's idea is foolish--but his language is so hard to understand that he could be saying the opposite of what he really feels. He tells Sue, "No, I will not bose as a model for your fool hermit-dunderhead" when he has every intention of posing for her. The reader is effectively surprised when it turns out that it really was the old German immigrant who dragged a tall ladder over to the nearby building in the dead of night and a howling storm, and created his masterpiece at the cost of his own life.

But, lo! after the beating rain and fierce gusts of wind that had endured through the livelong night, there yet stood out against the brick wall one ivy leaf. It was the last on the vine. Still dark green near its stem, but with its serrated edges tinted with the yellow of dissolution and decay, it hung bravely from a branch some twenty feet above the ground.