Spoilers: (Highlight to reveal)
Let me start by saying this is copied from reddit (link below) So take it with a grain of salt. Some of it seems to make sense. IF this is true I would be ok


http://www.reddit.com/r/MortalKombat


I have family in Chicago and one of it's member (my cousin) has a good buddy who has something do to with MKX. They are drinking often together and talking about many topics including jobs. So they were talking about MKX eventually.Here is what this guy told my cousin. Characters with quick description. Don't know is it true or only joke but sounds awesome :
Already revealed characters:
Sub-Zero - both brothers. Each have seperate chapter in story mode. Kuai is after Sektor and restoring Lin Kuei. Bi-Han is after Quan Chi.
Scorpion - Undead Spectre. After learning the truth about his clan his mission is to kill Quan Chi.
Kitana - Back from the dead. Neutral. She is against Kotal Kahn, Mileena and Raiden.
Quan Chi - Evil mastermind. His plan is to summon Balor who will help him ressurect MK champion Liu Kang as his revenant.
Kung Lao - Back from the dead. Cousin of young archer Kung Jin. He raised him.
Reptile - Kotal's ally. Loyal to everyone who has power to restore his kind.
Ermac - at first Kotal's ally, with story progress turns evil. Has Shao Kahn's soul.
Kano - Black Dragon clan leader. Has his own agenda, unwittingly helps both sides.
Raiden - Earthrealm protector. This time Raiden must help new generation of Special Forces.

Revealed Newcomers :
D'Vorrah - Kotal's right hand. She got eventually enough of his politics and betrays him.
Ferra/Torr - Deadly duo from realm merged with Outworld long time ago.
Kotal Kahn - New emperor of Outworld. Wise and strong leader. Dethrones Mileena.
Cassie Cage - Sonya Blade's and Johnny Cage's daughter. Leader of new Special Forces unit. y like father and stubborn like mother. In love with Takeda. Takahashi
Takeda - mild-mannered new member of Special Forces. Cassie Cage's love interest. Son of Kenshi and Hanzo's apprentice.
Jacqui - daughter of Jax and new member of Special Forces. Shares personality with her father.
Kung Jin - Young cousine of Kung Lao. He treats him like father and mentor.
Erron Black - Mercenary from Earthrealm and Sonya Blade's father. He is founder of Black Dragon. Real name : Salazar Blade.

Unannounced returning characters :
Mileena - only living heir of Outworld throne. Becomes Empress of Outworld but quickly dethroned by Kotal Kahn.
Shinnok - fallen Elder God and Netherrealm ruler. When QUan Chi retreats with his allies, Shinnok is defeated by combined strenght of Johnny Cage, SOnya Blade and Raiden during Netherrealm War and imprisoned in talisman.
Fujin - God of Wind. His mission is to spy Quan Chi and inform Elder Gods of his plan.
Liu Kang - Back from the dead by Quan Chi's magic. Now scarred and seeking revenge. Final boss of the game. Has only 2 variations - Boss form and regular form that can be playable after you unlock him.
Kenshi - Blind swordsman and father of Takeda. Member of Special Forces. Works with Johnny and Sonya. Later he helps Scorpion.
Jax - Back from the dead. Father of Jaqueline. Special Forces head-figure.
Sonya Blade - Wife of Johnny and mother of Cassandra. Her relations with family are hard. she is major of Special Forces now.
Johnny Cage - Husband of Sonya and father of Cassandra. Former movie Star, now second-in-command Special Forces officer and after Netherrealm War Earthrealm Champion.
Rain - Edenian Demigod almost killed by Kotal. He's back to revenge his failure. Major ally to Mileena's rebel army. Baraka - General of Tarkatan's army. Mileena's ally. He is leading his race to war against Kotal's Outworld army in Civil War.
Reiko - At first Mileena's ally that want to eventually betray her but after his plan flop he decide to take Outworld throne by himself using his mastermind.
Frost - Cryomancer from Earth. Orphan. She finds her way to Outworld via portal where she mets younger Sub-Zero and becomes his apprentice and Lin Kuei member. Rival to Cassie Cage.
Shang Tsung - Back from the dead. Has new body now. More of anti-hero now. Kenshi's rival.
Noob - Dark entity. After events of Mortal Kombat (2011), It is separated from elder Sub-Zero (Bi-Han) by Soulnado. Has new form, isn't ninja anymore.
Sektor - Grandmaster of Lin Kuei. Later attacked by younger Sub-Zero and defeated. After that he escapes and forms his own clan.

Unannounced newcomers :
Balor - supernatural being. New character in series inspired by Irish mythology. God of blight. He is called as blood demon. New Sub-boss.
Hanzo Hasashi - Scorpion's human form. He regained control over his body after Netherrealm War and intense training with his friend Kenshi. True mentor of Takeda.

Unlockable characters:
Shang Tsung Shinnok Noob Liu Kang Hanzo Hasashi
Pre-order DLC Goro - He is founded and healed by QUan Chi after he lost to Kotal. He is now his loyal servant.

DLC (kombat pack) NRS are making DLC's in order. So if they don't finish 1st character they don't start another. Jason is now revealed. My friend only heard rumours about that. 1st was movie guest character - and we know it is Jason. Jason Voorhees from Friday the 13th remake. They can't use Jason X (uber Jason) or anything from old movies as variations unfortunately. 2nd and 3rd characters will be characters that are spotted in-game, so either cameo or NPC. Most likely popular characters that are dead like Kabal, Jade , Cyrax or Smoke. Strong rumor about Tremor. Devs are impressed by his fans. And finally fourth and last DLC character from 1st kombat pack will be another guest character. It's either game or movie. Could be Talion from Shadow of Mordor or Predator. Maybe something else but this two are talked the most.
NPCs and cameos or mentioned in-game: Cyrax, Bo Rai Cho, Smoke, Li Mei, Havik, Kintaro, Sheeva, Sindel, Stryker, Nightwolf, Moloch, Tanya, Kai, Skarlet, Shao Kahn and basically almost everyone with few exceptions :P
Liu Kang is not regular MK boss. He hasn't cheap damage. He is very comboable and fast. He can basically string almost everything. On highest difficulty level one mistake and you can take very long combo. Balor is said to be VERY BIG, probably the biggest character ever in MK. Torr or Goro are said to be small compared to him. Words are that arcade ladder is Liu Kang boss form at the top, next is Balor and then is Shinnok who is replacing Shang Tsung in this role, then regular fighters, amounts depends on difficulty.