It's a testament to the scope of Tolkien's achievement that three very important minor characters haven't yet been mentioned: Arwen (who did get a brief mention in an above post), Elrond & Galadriel! They are all minor characters in the sense that they hardly appear in the book at all, and when they do it is to provide help or support to the major characters. They undergo no character development of their own, and are not real hindrances to the journey, nor do they play hugely 'active' roles in the Quest (more on that below).
Well, OK, as I write that I realise by my own definitions I shall have to take Galadriel off the list, perhaps
What is so great about their characterisation, however, is that while they are minor characters in the plot/story/narrative they are very important parts of the thematic concerns of the work. In this respect, Tolkien is comparable to Shakespeare (I DO hope you are reading this post Kransha!) who frequently uses characters for brief appearances in the plays, and yet without whom the whole fabric would fall apart (Caliban in
The Tempest for example, or Mercutio in
Romeo and Juliet).
Elrond is there as a repository of memory, and as a reminder of the huge sweep of history that the War of the Ring culminates. He is part of that history, and so the Quest to destroy the Ring could only begin from his home.
Arwen's thematic importance as the fulfillment and complement to Aragorn's story is so obvious (and has been dealt with so much in other threads) that it hardly bears comment here. And yet she hardly EVER appears in the book itself -- a minor character indeed, but hugely important.
Galadriel (who I will still put in my list) is the very embodiment of WHY the Ring must be destroyed. To have people talking all the time about how dangerous it is, is one thing: to actually have a Noldor tempted by it -- and to see the potential danger that she presents to the world should she take it. . .brrrrrrr
One thing that occurs to me only as I write this: the more powerful a character is, the more minor a role he or she is accorded. Only makes sense when the book is dedicated to foregrounding the hobbits. The great irony of the book revealed! -- the minor characters of the history of Middle-Earth are the major characters of LotR, and vice versa.