What are the differences and similarities among the characters of "The Last Leaf" by O. Henry?

All three of O. Henry's characters in "The Last Leaf" live
in the quaint Greenwich Village where "the art people soon came prowling."  But, while
Sue and Johnsy are young, aspiring artists, old Behrman is an effete artist, who has
"always been about to paint a masterpiece, but had never yet begun it."  Instead, he
subsists on income he earns by posing an artist's model with his impish body and "head
of a satyr."  Yet, all of them have their dreams;  Behrman his masterpiece, Johnsy has
dreamt of painting the Bay of Naples, and Sue perseveres in her drawing in hopes of
recognition.

Certainly, there is a bond among them.  When
Johnsy falls ill, Sue does everthing she can to encourage her friend's will to live. She
speaks positively to her friend, 

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"Dear! dear!...Think of me if you won't think of
yourself.  What would I
do?"

Behrman is angered that
Johnsy would despair so easily, but, of course, he cries; then, his efforts to save her
from succumbing to death are heroic.   Indeed, it is the bond among them that saves
Johnsy, with Behrman becoming the sacrificial victim of age who preserves youth by
finally painting his "masterpiece."