Halbarad was minor-yet-important. He brought with him some of Aragorn's credentials for the Kingship (Arwen's banner), without those he'd not have been able to claim the throne.
Arwen's role in the books (not her inflated role in the movies) is relatively minor, though she also sends Aragorn some needed "king equiptment" by way of Galadriel, namely the brooch which gave him his name Elessar.
The Wood-Elf guards who took pity on Gollum and ended up letting him escape play a major role in the story, in that without them, no final drop over the edge at Mount Doom. Same goes for Haldir, Rumil, and Orophin at Lothlorien when they hesitate to kill an unidentified shadow in the trees.
Snowmane is another one. Without Theoden's Bane would Eowyn have stood up to the Witch King? Doubtful.
Sophia
PS. Eeek! I almost posted this as a new thread, not a new post!
Thank goodness for the preview button!