Shao Kahn

Shao Kahn is the Emperor of Outworld and the final boss in almost all of the Mortal Kombat games.  He made his first appearance in MK 2 and has since been the major boss. All those who participate in the Mortal Kombat tournament and win must verse Shao Kahn. Shao Kahn's goal is to conquer Earthrealm and rule as leader of all the realms. I have never played as Shao Kahn as he is unplayable in MK9 apart from the challenge tower but even still I have not played as him. However in terms of him being a boss, I am now playing through the story a second time and he is horrifying yet again and quite possibly the worst boss I have faced in all of gaming.

Shang Tsung

Shang Tsung is Shao Kahn’s most loyal and trusted accomplice in the entire series and he has been since the very first game in the series. Not once in the series has Shang Tsung ever disobeyed or gone against Shao Kahn. He has always done exactly as the emperor has commanded. As I stated, he made his first appearance in the very first game to which he was a boss, however was not a playable character until the second game. I have not played much of Shang Tsung but he is a very painful boss to face in the main story of MK9. 
His fatalities are: 
Bang Bang: Down, Back, Down, Forward,  A/X
Identity Theft: Down, Down, Back, Down, Y/Triangle

Raiden

Raiden is the God of thunder for Earthrealm and its protector. He is said to be the most powerful character in the entire Mortal Kombat universe. He is possesses the powers of a god such as teleportation, flying and the control of lightning. He also has the ability to see foresee the future thus he is able to change the events of history. He made his first appearance in the first MK game and he is the main protagonist in Mortal Kombat 9. I find him to be a rather difficult character to play but he is also the final character to play as in MK9 and he is vital to learn to play if one is to win the game.
His fatalities are:
Just a scratch: Down, Forward, Down, Forward, X/Square
Transplant: Back, Forward, Forward, Down, B/Circle

 

Sheeva

Sheeva is another one of my least played and least liked characters in the game. When versing her NPC, there is a constant spam of her stomping attack which will end up killing you very quickly. She made her first appearance in Mortal Kombat 3 and has always aligned with Shao Kahn. Sheeva was always a playable character even though she was considered to be a sub-boss.  She was the personal bodyguard to Sindel but has since served Shao Kahn.
Her fatalities are:
Lend a hand: Forward, Back, Forward, Back, B/ Circle
Stripped Down: Forward, Down, Down, Forward, X/Square

Kano

Kano is another of the original characters that made his first appearance in Mortal Kombat 1. He is a member of the Black Dragon and was the first member to appear in the series. He wears a metal plate on his face due to an injury he suffered at the hands of Jax, however this took place before the events of the entire series meaning he has had the plate through the entire series. He is the main antagonist to Jax and Sonya but still joins forces with Shao Kahn in the fight against Earthrealm. I have only played Kano on a few occasions and he seems like a good character but a lot of his combos and super moves take a lot of time to do and he can also be quite slow.
His fatalities are:
Eat your heart out: Down, Down, Forward, Back, B/Circle
Heartbreak: Back, Down, Back, Forward, X/Square

Skarlet

Skarlet is the newest of all the Mortal Kombat characters. She is a DLC character in Mortal Kombat nine and however was another subject to speculation in Mortal Kombat 2 when she was said to have appeared as a glitch for Kitana, much the same as Ermac was for Scorpion. Her alliance is to Shao Kahn however in an Kahn's arena in during any fight not using her, she is shown to be chained to 2 posts as Kahn's "prisoner." Skarlet is a very enjoyable character to play but for people who are against a lot of gore, I suggest not using her fatalities.
Her fatalities are: 
Blood bath: Down, Back, Down, Down, Block
Make it rain: Forward, Back, Down, Down, B/Circle

Quan Chi

Quan Chi is another of my most hated character in the Mortal Kombat series. He has possibly the worst move set of any other character however uses magic rather well making him an annoying character to face. He made his first game appearance in Mortal Kombat Mythologies: Sub Zero but was not playable until Mortal Kombat 4. Quan Chi is the most powerful sorcerer in the MK universe and only seeks to conquer Earthrealm and all other realms. He has alliance only to those that are strong and look like they will win.
His fatalities are:
Beat Down: Forward, Forward, Down, Down, X/Square
On your knees: Down, Forward, Down, Forward, B/Circle