The narrator of The Name of the Rose is Adso of Melk. He is a young novice when the events occur in the story. He's naive and open-minded about the events. He's honestly curious, wanting to know things because he lacks the knowledge. His motives are pure and innocent because he's young, but he gets impatient and frustrated at times. He can be compared to Watson in the Sherlock Holmes mysteries. He assists William of Baskerville, the detective in the story.
Foucault's Pendulum is also narrated in first person. The narrator is Casaubon, an older man much like the old sailor in "The Rime of the Ancient Mariner". Casaubon's perspective of the characters and events is limited. His knowledge is vast, but there are still gaps in what he knows and understands. He refuses to admit his limitations, thinking his knowledge is omniscient instead of limited. Therefore, his narratiion is also limited. Casaubon becomes frustrated when he can't figure things out or when a piece of the puzzle is missing. He never quits trying to unlock the mystery, however, of the secret societies that offer the promise of unlimited knowledge and power.
Casaubon is the main detective in the book whereas Adso of Melk assists the main detective in The Name of the Rose. The narration differs because Adso is telling the story from a young man's point of view, but Casaubon is an older man who looks at things differently. Casaubon isn't an innocent narrator like Adso.