When we are introduced to the character of Behrman, O. Henry writes that he "regarded himself as especial masiff-in-waiting to protect the two young artists in the studio above," which means that he felt like he was a protector to Susan and Johnsy. This description of him being a protector foreshadows his later role in saving Johnsy's life. He expresses chagrin at Johnsy's fanciful and foolish fascination with the last ivy leaf and grumpily agrees to go up with Sue, to pose for her. When he gets up there, "In there they peered out the window fearfully at the ivy vine".
The next morning, the last ivy leaf stays clinging to the vine all that day, and through the night, even though previously, they had been dropping rapidly. This hints at the fact that the leaf is "different." As Johnsy starts to recover, Sue tells her that Behrman is very ill, and that "He is an old, weak man, and the attack is acute. There is no hope for him." His death is briefly foreshadowed here. Later she tells her that earlier they had found "His shoes and clothing were wet through and icy cold". We learn this right before Sue mentions that he had painted the leaf.
All of these tiny little clues can foreshadow the final sacrifice that the kindhearted Behrman made to help save Johnsy's life.