As you know from the reading the novel, it relies heavily on real officers from America's Civil War. So, it is rather unusual that the author MichaelShaarachose to include a fictional character--Buster Kilrain. Kilrain's last name is a play on words of sorts or a contraction of the novel's title Killer Angels. The "kilr" at the beginning are a contraction of "killer," and the "ain" represents "angels." Shaara most likely chose the name simply to be clever, but his reasoning for adding a fictional character to the novel is unclear. Perhaps because he was writing a fictionalized, but extremely accurate, version of an historic event, he added Kilrain's character to emphasize that at the end of the day his novel is fiction.
Regardless of his reasoning, Maara's son, Jeff, who continued his father's legacy by writing a prequel and sequel to The Killer Angels, believed Kilrain to be important enough to include in his works. He even includes background information on Kilrain, such as his birthplace (Ireland) and the reason for his demotion (a drunken assault on an officer). Kilrain dies in Killer Angels but not before being promoted.