Former hackers hired by either the government or a corporation like Majisoft to stop hackers from infiltrating their systems. Majicians who are able to transmit spells through Majispace via computers. Soldiers who are able to actually immerse their brain patterns in Majispace. They are the most efficient defense against hackers, but only the government can afford them.
Knights are either soldiers for the government or a guild.
Anyone can be a vigilante. It's simply someone who feels a strong sense of justice or was wronged by crime in some way. In either case, they felt a need to take the law into their own hands. In addition, they require a license to opperate legally. Needless to say, they do not feel a strong sense of loyalty to the government.