(Click the character infographic to download.)
He may as well be called Jeeves since he’s the ultimate butler—polite, loyal, and knowledgeable about his master:
"O, sir," cried Poole, "do you think I do not know my master after twenty years? Do you think I do not know where his head comes to in the cabinet door, where I saw him every morning of my life? No, sir, that thing in the mask was never Dr. Jekyll—God knows what it was, but it was never Dr. Jekyll; and it is the belief of my heart that there was murder done." (8.43)
Seriously, he claims to know Dr. Jekyll’s voice and footstep (and he probably does, seeing as he’s been with the guy for twenty years).
Poole is our old reliable; he isn’t much of a three-dimensional character himself, but this is so we can simply get information from him without having to question his motivations, reasoning, etc. He’s a tool for the author, in other words.