Your question identifies the difficulty that we have as readers in identifying the narrators of this prize-winning story. You are right in noting that the story is told from the perspective of two characters, and one of these is clearly identified as being Yunior, the friend of Oscar who only shares a room with him because of his friendship and affection for Lola, Oscar's sister.
The introduction to the story, as we come to realise later on in the novel, is written from the perspective of Lola, Oscar's sister. Note what she writes about the book she is writing:
Even now as I write these words I wonder if this book ain't a zafa of sorts. My very own counterspell.
The important concept of fuku, which is the equivalent of a curse that cannot be shaken and which Lola believes haunts her family and herself, haunts Lola, and indeed, it "has its fingers round my throat." Her attempt to exorcise the fuku is through recounting the story of Oscar's brief wondrous life and how he lived it.
Therefore the unnamed narrator is not Yunior, and the tale is told from the perspective of two characters: Lola and Yunior.