I think that Okeke is the main character in Achebe's short story.
Okeke's reaction to Nnaemeka's and Nene's marriage is critically important to the story. Okeke's emotions define the plot of the story. At the outset, he is a doting father, excited about the prospect of arranging Nnaemeka's wedding. Then, he becomes the disappointed father who offers arguments against his son's love for Nene. Okeke becomes the confused elder who turns to the other elders for guidance, as a response to his son's actions. Finally, he becomes the wrath- filled, embittered father.
In this form, he cuts out Nene's head from the wedding pictures that his son sends and writes angry letters to his boy, disavowing any connection between them. Achebe constructs the story with our focus on this old man and how or if he will ever come to any reconciliation with his son and daughter in- law.
The emotional climax of the story rests with Okeke's reaction to Nene's letter, which shakes the old man. He is thrown into a world of regret for the time he has spent steeped in his own anger. The story's ending is haunting with Okeke's vision of his grandchildren being left out in the unforgiving weather, "standing, sad and forsaken... shut out from his house." The final words of the story feature Okeke fearing his own mortality and not making right that which he made wrong.
Okeke initiates the action of the story and illuminates its depth because the reader sees the full extent of his growth. We hope that Okeke does right after doing so much wrong, and as a result, our empathy for him is what makes him the story's main character.