In "Things Fall Apart", Okonkwo is a traditional, "masculine" man who holds a place of power and respect among his people, the Umuofia. Ikemefuna is a boy from a rival tribe who was given to the Umuofia as part of a peace settlement. Ikemefuna lives with Okonkwo and becomes, essentially, a part of his family, but it is decided by the village oracle that the boy must be killed. Okonkwo condones and participates in the killing, largely out of a fear of being seen as weak if he does not participate. Ikemefuna's death haunts him and seems to bring bad fortune upon him and those around him.
"Things Fall Apart" is often treated as a tragedy, and Ikemefuna serves to initiate the tragic aspects of the plot; Okonkwo's "fall" from success and power. Ikemefuna's death points to a character flaw in Okonkwo, and it is acting upon this flaw that begins the tragedy. Okonkwo is preoccupied with maintaining his image, and this hubris leads him to ignore the warning of one of the elders not to participate in the killing. Thus, Okonkwo fulfills the role of a typical tragic hero, in that his own mistake or character flaw brings about his undoing; Ikemefuna brings out Okonkwo's tragic flaw. As heartless as it may sound, Ikemefuna himself isn't all that important - what is important is the role that he plays in driving the plot forward by forcing Okonkwo to make an unwise decision. Ikemefuna's influence is largely upon the story itself, not necessarily the characters.
In my links I have included the subject "hamartia" - this is Aristotle's term for a character's tragic flaw or error in judgment. Ikemefuna, as a servant of the plot, reveals Okonkwo's errors in judgment by creating a circumstance in which his traditions of masculinity and violence actually serve to diminish his power and reverse his fortunes.