Yunior’s writing about Oscar is characterized by hyperbole. For example, in Chapter 4:
These days I have to ask myself; What made me angrier? That Oscar, the fat loser, quit, or that Oscar, the fat loser, defied me? And I wonder: What hurt him more? That I was never really his friend, or that I pretended to be?
Yunior frequently uses exaggerated language like this to describe Oscar's lack of social success. Here, Yunior is recalling a moment in which his friends were making fun of Oscar. This hyperbole reminds readers of his unreliability, as his narration is colored by his emotions toward Oscar. Because of his personal relationship with Oscar, he can’t describe him objectively.
Clearly, Yunior loves Oscar a great deal—he’s written an entire biography of him. So this language is surprising, as most people don’t refer to a close friend as a “fat loser.” This hyperbole imbues Yunior’s narration with a sense of bitterness. On the page, it’s channeled toward Oscar in this mean language. But the bitterness comes across as anger toward the forces that caused Oscar to die.
Additionally, Yunior’s hyperbole is simply a part of his storytelling. Like Díaz, Yunior is telling a story, and his hyberbole makes it vibrant and funny.