How does Achebe indicate the importance of any two characters in his novel Things Fall Apart?
In a way, the answer to this question depends on what you consider "important." The character's importance to the novel, and whether a character is considered a protagonist, depends on the amount of space the novel devotes to the character, the degree to which the character is critical to the plot, whether the character is a viewpoint character (whether we see events through the character's eyes), and the degree to which the character influences other characters in the novel. Okonkwo is generally considered the protagonist of the novel by these criteria, as the plot revolves around him. Ikemefuna is important to the structure of the novel because he presents the protagonist with one of his major moral choices, and thus is a crucial component of the plot.
If by important you mean what qualities made people important within the society described by the novel, we find multiple sources of power, some rooted in material and some in spiritual influence. Okonkwo gains importance due to his physical strength, his skill as a warrior, and his wealth; his two barns and three wives are material emblems of his success. Chielo, on the other hand, has spiritual rather than material power in her role as a priestess of Agbala and Oracle of the Hills and Caves. She is consulted before people make important decisions and when she says it is necessary to sacrifice Ikemefuna, Okonkwo does so.