Guan Yu is a historical personage recorded in the "Records of the Three Kingdoms" which was written in the 3rd century and is considered a historical work. The "Romance of the Three Kingdoms" was written in the 14th century and is a fictionalized novel.

Here is a link to Guan Yu's Biography in the SGZ (chinese acronym for San Guo Zhi the work translated as "Records of the Three kingdoms") http://kongming.net/novel/sgz/guanyu.php

As you can see from the account, Guan Yu's basic character traits are already present.

Perhaps someone else can chime in with more detail on Guan Yu's further development as a literary figure but my understanding is that he figures prominently in a several of Yuan dynasty plays and a few earlier folk traditions before being incorporated into the Romance of the Three Kingdoms novel in the 14th (or perhaps 16th) century.

In any case, Guan Yu is in most respects a historical figure and not made up.

The history of Guan Yu has been covered properly.

To answer the second part of your question, Guan Yu was worshiped and revered before the novel was written. In general, it was important for people of Ancient/Classical China to know the classics and the history of the country so they definitely knew of Guan Yu and other generals of the Three Kingdoms. The deifying of Guan Yu began during the Sui Dynasty well before the book was written, with temples being built in his honor by the Tang Dynasty.

Today, he is pretty much considered a god. You might've heard that he is considered the "God of War". Here is an interesting article on how protesters of Hong Kong are worshiping Guan Yu during their protests. I haven't studied much on rituals or worship processes involving Guan Yu primary because I do not know how to read Chinese, however I always rely on these people to help me out