Ninja (jap. 忍者, singular ninja) are a class of warrior that use stealth and cunning, combined with physical prowess. Refer to Wikipedia:Ninja for more information.
There have been two examples of Ninjas shown thus far thru Megatokyo: Junpei and his master (who is unnamed, and appears only in no. 639). They are known to accept contracts from government interests (see also no. 10, and strips 213 and 215); it is not clear whether they also accept the from private interests. It also appears that one of their jobs is challenging foreigners without passports to Mortal Kombat battles.