A Wizard of Earthsea
Pechvarry, his wife, and their son Ioethe
Ged's time on Low Torning teaches us an important lesson about magic: it's good for stopping dragons, but it's less good for saving little boys who are too feverish. That is, Ged can take care of Yevaud the dragon (thanks to Yevaud's true name), but little Ioethe is too close to death to save.
So the Pechvarry/Ioethe part teaches us more about the setting – we see how magic works here and learn about the importance of the Balance. That is, it's not explicitly stated, but we can extrapolate and say that bringing someone back from beyond death is not in accord with the Balance. (Even if Ged could do it, which he doesn't seem to be able to anyway.)
Pechvarry, his wife, and their son Ioethe are also really important for the plot: Pechvarry becomes a good friend to Ged, and Ged's experience with Ioethe is how the shadow finds him. Which is kind of funny if you think about it. Pechvarry and his family both make Ged more comfortable in Low Torning, and yet they also make it impossible for him to stay there.