In the Movies, Haldir leads a contingent of Galadhrim to the aid of the Rohirrim at the Battle of Helm's Deep, where he and his people all die without really affecting the battle at all. In the opinion of many Tolkien purists, some PPCers among them, this was a pointless and incredibly stupid move on Peter Jackson's part. Although it did have some nifty Elvish battle music.
In Badfic Edit
In badfic, Haldir is often the best friend and/or Love Interest of a Sue. Alternately, he may be her father. Some authors claim that he is the rightful king of Lothlorien and his rule was usurped by Galadriel. Others ignore Celeborn entirely and claim that Haldir and Galadriel are married.