We aren't quite sure what type of book JK Rowling is writing in Harry Potter. She started out writing what many people assumed would be a typical Young Adult fantasy novel, but as the story progressed, it seems she is writing either a bildungsroman or a philosophical work. She also seems to be writing something akin to a spy novel, with the plots and schemes of various characters and factions becoming more convoluted as the story progresses.
This may be why Kingsley Shacklebolt is an important character in the Harry Potter series. His overall importance as a subjective personality in the book may not be very large (we don't know just yet), but he is important for other reasons. Two of those reasons may be his commanding yet serene personality in the somewhat high strung Order of the Phoenix, and the fact that he is one of the highest ranking wizards we have met so far that is black. But the real reason that Kingsley is important is he seems to be one of the most involved characters in the spy game that we meet. When we first meet him, in Book V, he has come to rescue Harry Potter from his aunt and uncle's house. Later on we find he is working for the Order of the Phoenix, and that his day job is as an auror for the Ministry of Magic, hunting Sirius Black, a hunt that he has no intention of completing, since he knows Sirius is innocent, and is indeed in the Order of the Phoenix with him. Later on in the book, he is sent to arrest Albus Dumbledore, an arrest he helps sabotage. So, throughout the fifth book of the series, he is a double agent, although he is a double agent for two sides that are not working against each other as much as they are mistrustful of each other.
We see Kingsley Shacklebolt only at the beginning of the sixth book. His boss, Rufus Scrimgeour, has been promoted from head of the aurors to Minister of Magic, and has given his protege an important job- going to work for the Muggle Prime Minister, and to spy on him and protect him. At this point, Kingsley has become a triple agent: he is apparently working for the British government, the Ministry of Magic, and the Order of the Phoenix, with his true loyalty going presumably to the Order of the Phoenix. It should also be noted that none of the organizations he is working for are working against each other as such, but are divided by miscommunication and mistrust.
What the presence of Kingsley Shacklebolt and his triple dealings suggest for the series is that there may be further revelations in the final book of various hidden allegiances. Or, as is the case often in Harry Potter, it may mean very little at all.