During MK1 and MK2, Reptile was Shang Tsung's personal bodyguard. Goro was Tsung's pupil. Kitana doesn't have a protector. I suppose, if anything, Mileena was sort of her watchdog to make sure she remained loyal to Kahn.

After Mortal kombat 2 Scorpion swore to protecked Sub-Zero to atone for killing his older brother.

I thought that during his part in the Deadly Alliance Shang Tsung gave the Oni Drahmin and Moloch, who had escaped from the Netherealm, an offer: after Tsung performed his part in the plan, Quan Chi, who the two despised for abandoning them in the infernal pits after promising to free them, would be theirs to do with as they wished.

So how can Drahmin and Moloch be Quan Chi's personal protector??

They protected him in the Netherrealm when Scorpion was out to get him.