For example, in a Season 3 episode, Mateo tells Jane, “Mummy, I want to read your book,” to which Jane responds, “When you’re older.” The narrator adds, “Much, much older. I’ve read it, it’s racy.” This could suggest that Mateo has now read the book and is retelling the story to the audience — racy bits and all!
Another clue comes in Season 4 when Jane asks Mateo if he’s ready for his first day of kindergarten. “Ah memories,” the narrator interjects, alluding to the fact that the conversation is one of his memories.
Mateo, Jane’s grandfather:
However, other fans seem convinced that Jane’s late grandfather, Mateo Sr., is the man behind the voice. This theory is backed up by the narrator’s thick accent, which Mateo Jr. does not have.
The narrator also has a lot of details about the Villanueva women, which Mateo Sr. would know if he was keeping tabs on them from the great beyond. One narration that supports this theory happens in Season 4 after Alba and Jorge are married.