Ikemefuna is a natural fit as a role model for Nwoye. He is well-liked by everyone in the family (including Okonkwo, although he'd never admit it), but he is special to Nwoye because he is a male figure that make Nwoye feel comfortable with masculinity. Until Ikemefuna arrives, Nwoye is abused and ill-treated by Okonkwo, until he's terrified to associate with anything feminine. But Ikemefuna has the balance of masculine & feminine that Okonkwo himself is lacking.
He was by nature a very lively boy and he gradually became popular in Okonkwo's household, especially with the children. Okonkwo's son, Nwoye, who was two years younger, became quite inseparable from him because he seemed to know everything. He could fashion out flutes from bamboo stems and even from the elephant grass. He knew the names of all the birds and could set clever traps for the little bush rodents. And he knew which trees made the strongest bows.
So he is a foil for Nwoye, slowly drawing him out of his shell with knowledge and companionship.
Ikemefuna knows "an endless stock of folk tales," and is able to tell them "with a new freshness and the local flavor of a different clan." Nwoye, who loves to listen to stories, is particularly impressed with this skill of Ikemefuna, and remembers the time spent listening to his adopted brother for the rest of his life.
Ikemefuna is two years older than Nwoye, and has a lively and intelligent persona. Ikemefuna fills a void in the younger boy's life, mesmerizing him with his knowledge of the world and his ability to identify wildlife and other elements of the world around them. Ikemefuna knows how to make a flute, and can find the trees that will make the best bows. Best of all, he is an animated storyteller, and he accepts Nwoye unconditionally. Because of Ikemefuna's influence, Nwoye blossoms in a way he never does under the judgemental eye of his father. The younger boy develops the confidence to participate more frequently in the gatherings of the men of the tribe. This has the effect of alleviating the rift that exists between Nwoye and his father, at least for a time.