A member of a Guild can buy an NPC wizard from the shop. These NPC's can ONLY be used in Guild events, such as Guild Wars. The NPC's of Guild's will fight each other, and for the most part, defeat each other, unless one has an obvious advantage over another. (Water vs Fire Magic). If Guild A has more NPC's than Guild B, Guild B's player mages will have to deal with Guild A's extra NPC's.
This is to give everyone something to buy, at all times, to continue making their guild stronger. There are no limits to how many NPC's a guild can have. The NPC's can have any type of non banned magic. It is suggested a guild does not only buy one type of NPC, as it could be countered easily.
When you purchase a Guild NPC, he/she starts off at D-rank. You can then later on rank them up buy purchasing something that ranks them up for a certain amount of Jewel. They are like the unnamed members of Fairy Tail/Phantom Lord during their war. They are the lowest grunts of the guilds.