who is the key character

Howie is dead on. Especially towards the end of the ROTK so many people could be considered the main character because Tolkien and PJ described the characters so well and they all of something huge for example Frodo, the ring bearer, Aragorn the returning king, Theoden, Eowyn and the other guy forgot his name, also starts with an E though the horse guy, Gandalf the white the most powerful/wise wizard you get the idea.

Eomer.................lol

As said in the Matrix (which is no doubt a reference to this amazing piece of literature):

The Path of the One, is made up by the Many.

Thus, no single one of them is a key character, rather all of them play a crucial role in the aiding of the destruction of Sauron.

yes very true...