A Wrinkle in Time
If you're going on a planet-hopping, time-traveling adventure through the cosmos, you couldn't do much better than Mrs. Whatsit for a guide. (Well, maybe Doctor Who.) Of the three Mrs. Ws, Mrs. Whatsit is the one most gifted at human speech, so much of the time she's the one directly tasked with providing guidance to Meg and the other humans. While her great power and superior knowledge enable her to give Meg & Co. the tools they need to rescue Mr. Murry, like a true mentor she stays out of the fray herself, but is there to offer congratulations once the quest is won.