Interesting question. I think the answer is just as different as the client. It is in the eye of the beholder. A realtor needs to adapt to the needs of the client. In my mind, the Realtor needs to use their expertise and experience to give the buyer all the information they need to make a good decision. They need to be honest, knowledgeable, and provide all information - good and bad to the client.
Realtors should be guides and sounding boards for their clients as well as a wealth of information and knowledge to answer any and all questions.
But I think the singular most important role of a Realtor is communication. They need to be available to clients when they have questions and need information. That means - not being a "yes" man to buyers or sellers and equally not being a hard sell. Exceptional service and working toward the client's best interest.