One early example of discrimination against Eriko’s transgender identity comes via a conversation between Mikage and Yuichi. Remember, the story is told from Mikage's point of view. Point of view is a literary device. When Yuichi tells Mikage that his mom was once a different gender, Mikage perceives that Yuichi could “barely contain his amusement.” Through Mikage’s point of view, the reader might come away with the idea that Yuichi sees his mom’s gender identity as a joke or spectacle. Such a viewpoint is discriminatory.
Another literary device that Banana Yoshimoto deploys to address Eriko’s discrimination is foreshadowing. Remember, right before Eriko is killed, she and Mikage have a conversation. Eriko explains to Mikage the necessity of despair and hard times. Without them, it’s hard for a person to appreciate joy. You could claim that this exchange prepares the reader for the fatal suffering that is on its way. Soon, Eriko will face hardship that she cannot overcome.
You might also want to focus on the paragraph that summarizes Eriko’s murder. You might be able to identify several literary devices in that one paragraph. One literary device could be hyperbole. Mikage describes Eriko’s murderer as “crazy” and “obsessed.” Perhaps if the murderer didn’t harbor such strong hate, he wouldn’t have committed the deadly act of discrimination.